Multi-way vectored interrupt capability
RELATED APPLICATIONS 
     The following U.S. Patent Applications filed on an even date with the instant application and assigned to the same assignee as the instant application are related to the instant application. 
     1. &#34;Hardware for Extending Microprocessor Addressing Capability&#34; by Thomas O. Holtey and Robert C. Miller, and having U.S. Ser. No. 000,304. 
     2. &#34;I/O Request Interrupt Mechanism&#34; by Thomas O. Holtey, and having U.S. Ser. No. 000,315. 
     3. &#34;I/O Priority Resolver&#34; by Thomas O. Holtey, and having U.S. Ser. No. 000,477. 
     4. &#34;Paging Mechanism&#34; by Thomas O. Holtey, Robert C. Miller and Kin C. Yu, and having U.S. Ser. No. 000,463. 
     5. &#34;Hardware for Remote Maintenance of Computer Systems&#34; by Thomas O. Holtey and Kin C. Yu, and having U.S. Ser. No. 000,314. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to data processing systems and more particularly to addressing apparatus for interrupting a computer program utilizing an interrupt vector which can provide the addresses of 64 unique function codes for directing the execution of any of 64 interrupt routines. 
     2. Description of Prior Art 
     In running a computer program there are in general two phases which normally alternate. First, instructions and/or operands are fetched to the central processor and then the instructions are executed. During the fetch phase, the processor fetches the next instruction to be executed from main memory, and it to one or more control registers where such further modifications for indexing, indirect addressing, and base addition as required may be performed. During the execution phase of an instruction, the processor decodes the instruction and the operation specified by the instruction is executed. 
     Most stored program digital computers have some means of interrupting the running program upon request either from an external or internal signal. There are a variety of reasons why it is necessary to interrupt a computer program, such as for input or output of information or for calling some routine for performing a standard routine such as maintenance, diagnostic, square-root, etc. 
     The simplest form of interrupt is to utilize an instruction which causes a halt or branch, such as a HALT or a GO TO, and RETURN subroutine which causes the CPU to go to another location in main memory to obtain the next instruction to be executed rather than continuing with the next sequential instruction of the program. However, before executing the new program or routine, the contents of any indirectly addressable registers such as program counters, accumulator registers or the like must be preserved so that control may be returned to the interrupted program at the point where it was interrupted. This entails additional time in unloading and reloading the program counter. 
     There are many variations to this basic interrupt function such as conditional interrupts, priority interrupts, and nested interrupts. See the following U.S. Pat. Nos. for typical prior art interrupts: 
     (1) Interrupt System, by H. D. Wise, having U.S. Pat. No. 3,226,694, issued Dec. 28, 1965. 
     (2) Digital Computer Having High Speed Branch Operation, by R. E. Packard, et al, having U.S. Pat. No. 3,408,630, issued Oct. 29, 1968. 
     (3) Priority Interrupt Monitoring System, by Ralph Benghiat, having U.S. Pat. No. 3,221,309, issued Nov. 30, 1965. 
     (4) Subroutine Call and Return Means for an Electronic Calculator, by Richard M. Spangles, having U.S. Pat. No. 3,693,162, issued Sept. 19, 1972. 
     (The above references are exemplary only and no inference should be drawn that this is the closest prior art to the invention.) 
     Generally, both instructions and data operands are stored for execution in various addressable locations in the same main memory. Moreover, the instructions may be further grouped in groups forming subroutines. This requires that address locations be assigned in advance when the program of a subroutine for a particular problem is computed. Moreover, if the subroutine is later modified, the programmer must keep track not only of the original allocations of memory but also of the later modifications. In order to make programming independent of the actual address locations in main memory, indirect addressing has been resorted to. By means of index registers, groups of data or programs may be used without assigning specific address locations to each word in the group at the time the program is compiled. Although the absolute address is later inserted in the memory location referred to by the command, it can be different each time the program is run, depending on where the group is placed by the executing program in main memory. Since the group may be indirectly addressable by a number of commands, the address of the commands remains unchanged and only the absolute address of the group is changed if the location of the group is modified. This technique generally requires special words known as descriptions in place of operand words or instruction words in memory. When an instruction causes an operand word to be read out of memory, it may find a data descriptor word which includes a base address of a group of words. This further includes information identifying it as a descriptor word instead of an operand word, information as to the length of the group of instructions or data, and information indicating whether or not indexing is required. 
     (See U.S. Pat. No. 3,938,096, issued Feb. 10, 1976, entitled &#34;Apparatus for Developing an Address of a Segment Within Main Memory and an Absolute Address of an Operand Within the Segment&#34;; see also U.S. Pat. No. 3,222,649, issued Dec. 7, 1965, entitled &#34;Digital Computer With Indirect Addressing&#34;. ) 
     Whereas it is desirable in an interrupt operation to have operands or instructions grouped in groups which may be placed anywhere in main memory, it is just as desirable that the addresses of the descriptors or other type indirect address words be fixed. This may be done by having a central location such as a segment or a table for storing such words. Both of these techniques are comparatively slow; one requires several accesses of main memory to access descriptors by indirection, whereas the other requires comparison of the command or function code word to the words stored in the table. Moreover, these techniques require substantial amounts of memory space. Where a microprocessor is used, such storage space and speed are at a premium. Accordingly, a hardware technique of interrupt by indirection to any one of several predetermined locations is needed which is fast and is miserly in the utilization of processor and main memory storage. 
     OBJECTS OF THE INVENTION 
     It is a primary object of the invention to provide an improved mechanism for interrupting a computer program executing on a computer system. 
     It is another object of the invention to provide improved hardware for addressing a computer memory system by indirection. 
     It is still another object of the invention which provides addressing hardware which modifies a base address by function code signals to generate address information in memory having variable addresses. 
     It is still another object of the invention to provide an interrupt mechanism for a computer system for use in a microprocessor. 
     SUMMARY OF THE INVENTION 
     The foregoing objects are achieved according to one embodiment of the invention by providing a multi-way vectored interrupt mechanism for interrupting an executing program using an indirect method of addressing for pointing to any of 64 interrupt subroutines stored in a memory. 
     Predetermined locations in memory store the high order and low order bytes of the interrupt vector respectively. When the location in memory storing the high order byte of the interrupt vector is addressed by the microprocessor, the high order position of the program counter of the microprocessor is provided with the high order byte of the interrupt vector. However, when the low order byte of the interrupt vector is addressed, the bits of the address are variously applied to different hardware in the system which disables the normal addressing mechanism of the system and enables an indirect addressing mechanism. The indirect addressing mechanism provides a selected address to one of 64 locations in the memory. The contents of this selected address provide a second address which is placed in the low order byte position of the program counter of the microprocessor, thus giving an absolute address of a desired location in memory where an interrupt sub-routine is stored. Thus, any of 64 interrupt subroutines may be quickly called with a minimum of space requirements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features which are characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, both as to organization and operation together with further objects and advantages thereof may best be understood by reference to the following description taken in conjunction with the drawings in which: 
     FIG. 1A is a schematic block diagram of the preferred embodiment of the invention. 
     FIG. 1B is a schematic diagram of typical addressing formats of the invention. 
     FIG. 1C is a map of the paging PROM. 
     FIG. 2A is a schematic diagram of a typical organization of the memory of the invention as determined by real address signals. 
     FIG. 2B is a schematic diagram of a typical organization of the memory of the invention as indicated by virtual address signals. 
     FIGS. 3, 4, 4A, 4B, 4C, 5A, 5B and 7 are diagrams of the preferred embodiment of the invention. 
     FIG. 6A is a flow chart description of portions of the invention. 
     FIG. 6B is a timing diagram of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1A, there is shown a logic block diagram of a preferred embodiment of the invention which also shows information flow and modification of the information for improved addressing. A microprocessor 101 is typically a type 6800 commercially available from Motorola Inc. and utilizes a 16 bit address bus 102 to address main memory 108. (The 6800 is described in &#34;The Complete Motorola Microcomputer Data Library&#34;, Series A, Motorola Inc. 1978 by Motorola Semiconductor Products, Box 20912, Phoenix, Arizona.) This provides an addressing capability of over 64,000 bytes of main memory 108. The formats of the instruction are shown on FIG. 1B. There are primarily two formats; one having an eight bit op code and an 8 bit (a) byte, whereas the other one has an 8 bit op code, and 8 bit (a) byte and an 8 bit (b) byte. In order to conserve space and cycle time, it is more advantageous to use only the (b) byte. Accordingly, in the schematic representation of FIG. 1A, the register 103 utilizes the first 5 high order bits 8, 9, 10, 11, and 12 to address the paging signal generator 105. (The paging signal generator 105 (PROM 300) is an integrated circuit memory chip of the type designated as 5610 and commercially available from Intersil Inc. The 5610 is described in the &#34;Intersil Semiconductor Products Catalog&#34; by Intersil Inc., 10900 North Tantau Avenue, Cupertino, Calif., published October, 1974.) The paging signal generator 105 stores 32 words which can be addressed by bits 8-12 of the (b) byte. Since 5 bits are utilized to address the paging signal generator, they can be utilized to address any of the 32 words therein. The internal circuitry of the generator 105 is such so that when the first 8 words (up to address 07) are addressed, signal CPGLIN is activated (i.e., goes low). When the next four words of the signal generator 105 are addressed (i.e., addresses 8-11,) then both signals CPGLIN and CPGDIR are activated. When the next location word 13 (i.e, address, 12) is addressed then all of the following signals are activated (i.e., go low), CPGLIN, CPGDIR, CPGCCB and CPGAD4. The paging signal generator 105 is enabled when a low output signal from the microprocessor 101 is present at its E input terminal. A low input signal is provided to the E input terminal of paging signal generator 105 from the output of OR gate 104 when all of its input bits 1-8 of (a) byte are 0 or low. These bits 1-8 of (a) byte are made 0 when it is desired to modify the 16 bit address provided by the (a) and (b) byte. Accordingly, when all the bits of the (a) byte are 0, a low signal results at the output of OR gate 104 which is applied to the E input terminal of paging signal generator 105 thus enabling it. When the paging signal generator 105 is enabled, one of the control signal locations 105a is addressed by bits 8-12 of (b) byte. When selected ones of these control signals 105a are active, (i.e. low,) the 16 bit virtual address 106 is modified to the real address 107 which then addresses main memory 108. If none of the control signals 105a are active, then the 16 bit address 106 is identical to the 16 bit address 107 and no modification occurs for addressing memory 108. (The mechanism for performing this modification will be discussed in detail in relation to FIG. 3.) Assuming, therefore, that control signal CPGCCB is active, then bit 11 of the virtual address is replaced by the bit in position α of CCB register 115 and bit 12 will be replaced by bit β in CCB register 115 to form the real address. If control signal CPGDIR is active, then bit 10 of the virtual address is replaced by bit D of channel register 114. If control signal CPGLIN is active, then bit 9 of the virtual address is replaced by bit M of the CH register 114, and bit 8 of the virtual address is replaced by bit H of the CH register 114. If control signal CPGAD8 is active, then bit 7 of virtual address is replaced by a one. Finally, if control signal CPGAD4 is active, then bits 4, 5, and 6 of the virtual address are replaced by a one. 
     The CE$U2U control signal output of the paging signal generator 105 is utilized to address the line number of a selected Universal Synchronous Asynchronous Receiver Transmitter (USART) 116, 117. (USART&#39;s are commercially available from Intel Corporation, and are of the Programmable Communications Interface type 8251A. The 8251A is described in the &#34;Intel Component Data Catalog 1978&#34; published by Intel Corporation, 3065 Bowers Avenue, Santa Clara, Calif. The CEIO2U control signal enables the I-bus 113 via bidirectional bus driver 111. (These bidirectional bus drivers are commercially available from Texas Instrument and are designated as type 74LS245. The 74LS245 is described in &#34;The TTL Data Book for Design Engineers&#34;, Second Edition, copyright 1976 by Texas Instrument. The CEIO2U signal permits communication from the I-bus 113 to the U-bus 112, whereas a CEU2IO signal permits communication from the U-bus 112 to the I-bus 113. The I-bus may have various registers attached for storing communications information. Some typical registers are the HI-Order Data Register 120, LO-Order Data Register 121, channel no. register 122, and status register 123. These registers communicate with the microprocessor via the I-bus 113 and the U-bus 112, and with main memory 108 via I-bus 113 and M-bus 109. In order for the various registers on the the I-bus 113 to communicate with main memory 108 and microprocessor 101, it is necessary to assign space in main memory for various lines and channels associated with any communication port. Referring therefore to FIG. 2A, it will be seen that real memory 200 has a portion of its area reserved for lines 0-3. Each line is comprised of 64 bytes and the total 4 lines 0-3 comprise the Logical Table (LCT) space. Each line 0-3 is furthermore subdivided into 2 channels of 32 bytes each. Accordingly, there are 8 channels of 32 bytes comprising 4 lines of 64 bytes, each of which comprise the LCT space. The next 256 bytes are reserved for Channel Command Programs (CCP) use. There are also 3 to 4K bytes which together with the unused space are reserved for Channel Command Programs (CCP). Below this space there is an additional 256 bytes reserved for the Channel Control Block (CCB). As with the LCT space, each line 0-3 is associated with one CCB of 64 bytes, each of which is subdivided into 2 channels of 32 bytes each. Below this is memory space reserved as firmware work space. Accordingly, it can be seen that each line 0-3 is associated with one LCT space and one CCB space, each of which is subdivided into two channels. Part of the addressing mechanism described supra with FIG. 1A addresses all of these memory spaces. However, to do this it takes two address bytes a and b since one address byte is comprised of 8 bits and 8 bits can address only 256 locations. Yet as can be seen from FIG. 2A, there are 768 locations (3×256) excluding the 3K/4K locations. These 256 locations are the most commonly addressed since communication of lines 0-3 must constantly be had with its LCT&#39;s, its CCB&#39;s and the firmware. It is very inefficient to utilize the 16 bit address which can normally address over 64,000 locations merely to address 768 locations, yet only one 8 bit address can address only 256 locations. This invention permits the 768 locations to be addressed by the first 5 bits 8-12 of the (b) byte 103 by permitting the modification of the virtual address of FIG. 2B as discussed supra. Hence cycle time and storage space are saved by this short form of addressing. 
     Referring to FIG. 1C there is shown the map of the paging signal generator 105 (i.e., Paging PROM 300). The map is self-explanatory. The address locations are shown in various numbering systems on the first 3 columns, whereas the last column contains the actual information stored at that address location. The fourth column designates the hexadecimal locations which have similar contents. 
     Referring to FIG. 2B, there is shown 256 locations in memory 201 reserved for virtual memory. The first 64 locations or bytes are numbered in decimal notation 0 through 63 and in hexadecimal notation 0 through 3F, and comprise the LCT of the current line used by the CCP. The next 32 locations or bytes, decimal locations 64-95 and in hexadecimal notation 40-5F, are reserved for the LCT of the current channel used by firmware. The next 8 locations or bytes denoted in decimal notation 96-102 and in hexadecimal notation 60-67 are reserved for the active CCB of the current channel. There is then an unused space and there are 3 eight byte locations reserved for the USART of the current line, the shadow USART of the current line respectively and the extension of LCT of the current channel. 
     A typical example will illustrate how the improved addressing scheme of the invention works. Assume, therefore, that location 5 of line 0 of virtual memory 201 is to be addressed. Accordingly, all of the bits 0 through 7 of (a) byte of register 103 would be 0 which would enable OR gate 104 and enable the paging signal generator 105. The next 5 bits, 8 through 12, would also be 0, whereas bit 13 would be a 1, bit 14 would be 0 and bit 15 would be a 1, thus giving the binary address 101 or decimal 5. The virtual address 106 would have also have bits 0 through 12 equal to 0 with bit 13 being 1, bit 14 being 0 and bit 15 being 1. Additionally, however, since bits 8-12 of the (b) byte in register 103 are 0, control signal CPGLIN would be active. (It was seen supra that if bits 8-12 were utilized to address the first 8 words in the paging signal generator 105, signal CPGLIN would be active or low.) With signal CPGLIN active, bits 8 and 9 of the virtual address 106 would be replaced by bits H and M respectively of channel register 114. Under our assumption, which initially was that we are addressing location 5 of line 0, bits H and M of channel register 114 would be 0 and accordingly bits 8 and 9 of real address 107 would also be 0. Thus the final real address would have bits 0-12 equal to 0, bit 13 would be a 1, bit 14 would be a 0, and bit 15 would be a 1, thus addressing the fifth location of line 0 of real memory. 
     To take this problem one step further, assume now that the fifth location in line 1 is now to be addressed. The bit contents of register 103 and virtual address 106 would be identical as in the prior example. However, since line 1 is now being addressed, the channel register 114 would have a 0 in its high order bit H and a 1 in its next order bit M. Accordingly, when signal CPGLIN is activated once again (since bits 8-12 of the (b) byte of register 103 are all zeroes), bit 8 of virtual address 106 would be replaced by bit H of channel register 114 which is a 0. Bit number 9 of virtual address 106 would be replaced by the mid bit M of channel register 114 which in this example is a 1, since line 1 is being addressed. Hence the real address 107 would have zeroes in bit positions 0 through 8, bit 9 would be a 1, bits 10-12 would remain 0, and bit 13 would still be a 1, bit 14 would still be 0, and bit 15 would still be a 1. Accordingly, now hexadecimal location 45 is addressed in real memory which is the fifth location of line 1. It can readily be seen by this reasoning that at location 5, line 2 or line 3 could be similarly addressed merely by substituting bits H and M of the channel register 114 for bits 8 and 9 of the virtual address 106 to obtain the real address 107. 
     Referring to FIG. 3, there is shown the detailed logic block diagram of the paging apparatus for improved mapping of virtual addresses to real addresses. First, a structural decription will be given wherein the various structures of FIG. 3 will be identified and tied into FIG. 1A where feasible; secondly, the operation of the structure of FIG. 3 will be described to show the various functions performed. It should be noted from FIG. 1A supra that the paging mechanism is designed to modify bits 4 through 12 of virtual address format 106 to provide the final real address 107 with bits 4 through 12 either modified or not in accordance with the signals presented. On FIG. 3 it should be noted that multiplexers (MUX) 302, 303, and 304 and driver 305 provide the output signals respectively on lines 302A, 303A, 304A, 305A and 305B which represent modified bits 8 through 12 of the real address 107. Multiplexer (MUX) 301 and driver 308 provide the output signals in line 301A, 308A, 308B, and 308C which represent bits 4 through 7 of the modified real address 107. Register 309 corresponds to register 114 of FIG. 1A and stores bits H, M and D and provides these bits as output signals on lines 309A, 309B and 309C. Register 310 corresponds to CCB register 115 on FIG. 1A and stores and provides the α and β bits as signal output on lines 310A and 310B respectively. PROM 300 corresponds to paging signal generator 105. As described supra, it provides the various signals for mapping the virtual address 106 into the real address 107. The map of PROM 300 corresponds to FIG. 1C. Drivers 305 and 306 are coupled with AND gate 311A to provide the real memory address bits 11 and 12. Register 311 is utilized to store various signals. 
     Each of these devices is commercially available from such manufacturers as Texas Instrument, Motorola, Intel and other semiconductor manufacturers in accordance to their universal designations as shown in Table I below: 
     
                       TABLE I                                                     
______________________________________                                    
Type of Device and   Commerical Identi-                                   
Numeral Designation  fication Number                                      
______________________________________                                    
MUX301, 302, 303 &amp; 304                                                    
                     74LS253                                              
Drivers 305, 306, 307, 308                                                
                     74LS241                                              
&amp; 370                                                                     
Registers 309 &amp; 114  74173                                                
Registers 115, 120, 121, 122                                              
                     74LS374                                              
123, 123A, 310, 311, 360, 361,                                            
370, 371 &amp; 372                                                            
AND gate 311A, 354, 358A, 371 &amp; 372                                       
                     74LS08                                               
PROM 300             5610                                                 
Decoders 351, 352, 353, 355, 356,                                         
                     --                                                   
357 &amp; 380                                                                 
Comparators 358 &amp; 358B                                                    
                     --                                                   
USART 116 &amp; 117      --                                                   
Bus Drivers 110, 111, 370 &amp; 711A                                          
                     --                                                   
______________________________________                                    
 
    
     The 74 series circuits are listed in &#34;The TTL Data Book for Design Engineers&#34;, Second Edition, copyright 1976 by Texas Instrument. The 5610 is listed in the &#34;Intersil Semiconductor Products Catalog&#34;, published in 1974 by Intersil Inc. 
     Referring once again to FIG. 3, the operation and function of the paging apparatus for improved mapping of virtual addresses to real addresses will be described in greater detail. As has been discussed supra with respect to FIG. 1A, when addresses 0 through 7 of PROM chip 300 are addressed the communication paging line signal (CPGLIN) becomes active by going low. This is shown on the Paging PROM Map of FIG. 1C, wherein the contents of the map in the first 8 positions are 01111111. Bit position 7 is 0, or low, which activates signal CPGLIN. This signal is then applied to input terminal 2ag and 2ah of multiplexers (MUX) 302 and 303 respectively in FIG. 3. The other input control signal to input terminals lag and lah of multiplexers 302 and 303 respectively is the logic 1 (LOGIC 1) signal which is wired to always be high. When signal CPGLIN is active (i.e., in the low state), it addresses input terminals lag and lah multiplexers 302 and 303 respectively which means that the signals on input terminals lg and lh will pass through as outputs on lines 302A and 303A respectively. By following the signals (CPGCNH) on input terminal lg of multiplexer 302 back to its source, it will be observed that it comes from the high order bit on line 309A of channel register 309. Similarly, following the input signal (CPGCNL) on input terminal 1h of MUX 303 back to its source shows that it comes from the middle order output line 309B. These correspond to bits H and M of channel register 114 of FIG. 1A. Accordingly, when the line paging signal (CPGLIN) is activated, the H and M bit of register 114, 309 is substituted for virtual address bits 8 and 9 on output lines 302A and 303A respectively. Conversely, when the line paging signal (CPGLIN) is not activated (i.e., high,) then the address bits 8 and 9 of the virtual address are not modified and are passed as is to the output lines 302A and 303A of MUX&#39;s 302 and 303 respectively. This occurs because when CPGLIN is high and with LOGIC1 always being high, input address 3g and 3h are addressed on MUX&#39;s 302 and 303 respectively. Input address 3g of MUX 302 is CADU08 which is interpreted as the communication address of the microprocessor bit 8. Input address 3h of MUX 303 is CADU09 which is interpreted as the communication address microprocessor bit 9. When input terminals 3g and 3h are addressed, this becomes active and permits the addresses on that terminal to pass through to the output lines of 302A and 303A of MUX&#39;s 302 and 303. 
     The next control bit for modifying the virtual address 106 from the PROM chip 300 is the directional bit (CPGDIR). The directional bit is the low order bit D in channel register 114 and on line 309C of channel register 309. The directional bit becomes activated when addresses 8, 9, 10 and 11 (decimal) of the PROM 300 are addressed (See FIG. 1C). Additionally when these bits 8-11 are addressed output signal CPGLIN also becomes active. Accordingly, in addition to the application of the CPGLIN signal to MUX&#39;s 302 and 303, there is an application of signal CPGDIR on input terminals 1d and 1ai of MUX&#39;s 301 and 304 respectively. With signal CPGDIR on input terminal 1ai of MUX 304 low, it makes no difference whether input signal CPGAD8 on input termial 2ai of MUX 304 is high or low, since under either condition either input terminal 0b or 2b (addresses 00 or 10 binary) are activated and the CPGCND) signal is applied to both these addresses. The origin of the CPGCND signal is from the output line 309C of channel register 309 which is the D bit of channel register 114 and channel register 309. Accordingly, when the directional bit CPGDIR is activated the number 10 (decimal) bit of virtual address 106 is modified in accordance to the contents of the D bit of the channel register 114 or 309. There is no effect of the CPGDIR signal on the 1d input terminal of MUX 301 unless the CPGAD8 signal is also activated. This is true because with signal CPGAD8 inactivated or high, only addresses 2e or 3e (10 or 11 binary) of MUX 301 can be addressed. They are both the same and represent bit 7 of the communication address of the microprocessor. However, when the CPGAD8 signal from PROM 300 is also activated (i.e., low,) then only address 0e or 1e (00 or 01 binary) of MUX 301 is addressed and becomes active; both these addresses have the logic signal 1 LOGIC1 applied which is permitted to pass to output line 301A of MUX 301 when both signal CPGAD8 and CPGDIR are active or only when CPGAD8 is active. 
     Hence with CPGAD8 active, bit 7 of virtual address is modified and forced to a one. 
     As described supra with respect to FIG. 1A, when the channel register bit (CPGCCB) is active or low, then bits 11 and 12 of virtual address 106 are replaced by channel bits α and β of register 115. Since register 310 on FIG. 3 corresponds to channel register 115, and bit CPGCCH on output line 310A corresponds to the α bit of channel register 115, and bit CPGCCL on output line 310B corresponds to the β bit of register 115, then these bits will replace bits 11 and 12 of the virtual address when the signal CPGCCB is active or low. Let&#39;s see how this happens. When the signal CPGCCB is activated, it is applied to the 11 input terminal of driver 306 and to the 1 terminal of AND gate 311A. Accordingly, driver 306 is enabled and the channel control bit signals CPGCCH and CPGCCL on output lines 310A and 310B are applied to terminals 1n and 0n, respectively of driver 306. They pass through to output line 306A and 306B of driver 306 and replace bits 11 and 12 of the virtual memory address. It should be noted that when the CPGCCB signal applied to input terminal 1 of driver 306 is low, it enables driver 306; but this same signal applied to the input terminal 19 of driver 305 disables driver 305. Hence the CADU11 and CADU12 signals on input terminals 24 and 25 respectively of driver 305 are not passed through to the output terminal 305A and 305B of driver 305, but rather are replaced by channel register 310 bits as previously described. Accordingly, it is seen that either driver 306 or 305 is enabled but not both, and either the channel register bits are passed through via driver 306 or the microprocessor address bits are passed through to the output via driver 305. 
     Finally, with respect to the virtual address modification, the modification of bits 4, 5 and 6 will be discussed. As noted supra with respect to FIG. 1A, this is accomplished via signal CPGAD4. When address 12 (decimal) or paging signal generator is addressed, all of the following signals become active. CPGLIN, CPGDIR, CPGCCB and CPGAD4. This is seen by referring to FIG. 1C where address 12 (decimal) contains the following 00001111. Hence bit positions 4, 5, 6 and 7 are low or active, and from FIG. 1A ref. numeral 105, these correspond to signals CPGAD4, CPGCCB, CPCDIR and CPGLIN respectively. It has already been shown how the first three signals modify the virtual address when they are active, and it will now be shown how the signal CPGAD4 modifies the virtual address and forces one&#39;s into bits 4, 5 and 6 of the virtual address. The CPGAD4 signal is applied to the enabling terminal 19 of driver 308. When driver 308 is not enabled (i.e., low,) then one&#39;s are forced for bits 4, 5 and 6 respectively. If it is enabled (i.e., high), then the microprocessor address CADU 4, 5, and 6 respectively will pass through. The reason for this is that driver 308 is a commercially available LS241 tri-state circuit which has pull up resistors for the signal applied. Accordingly, if a low signal such as CPGAD4 is applied, it does not enable driver 308 and the output signals are pulled up to +5 volts making it a logic 1. On the other hand, when CPGAD4 is not active it is high, thus it enables driver 308 and permits the address signal on input terminals 1k, 2k, and 3k respectively of driver 308. 
     Not only does the paging signal generator 105 on FIG. 1A and its equivalent PROM 300 on FIG. 3 generate signals that can more efficiently address memory 108, but it further generates signals which provide more efficient addressing and communication between main memory 108, the microprocessor 101 and various registers and peripherals attached to the I-bus and U-bus. This communication between various devices such as register, memory to register, utilizing the U-bus and the I-bus is initiated by activating signal CEU2I0 (i.e., signal CEU2I0 is low and represented on FIG. 1C by a zero). It should be noted that signal CEU2I0 is bit position number 1 on paging signal generator 105. Now referring to FIG. 1C, which is the map of paging signal generator 105 and its equivalent PROM 300, it should be noted that there are 3 addresses where there is a zero stored at bit position number 1. They are decimal locations 18, 21 and 22 or virtual address hexadecimal locations 90, A8 and B0. Accordingly, when any of these locations of paging signal generator 105 or PROM 300 are addressed by the microprocessor 101, signal CEU2I0 will become active or low. Signal CEU2I0 initiates the communication process and controls the enabling of the bus driver 111 on FIG. 1A and 711A on FIG. 7 by means of an OR gate 911B. It is also applied as one input to AND gate 354 of FIG. 4 and guarantees a pulse out after the data becomes valid on the bus. 
     Referring to FIGS. 3 and 4, a signal CEU2I0 is generated at bit position 1 of PROM 300 when it is desired to enable the I-bus driver 111 of FIG. 1A or its equivalent the I-bus driver 711A of FIG. 7 to cause data to be driven from the U-bus to the I-bus and written into either the channel register 114, the CCB register 115 or the S register 123A. The S register equivalent 700 appears in FIG. 7. Bus driver 711A is enabled by signal CEIBUS, the output of an OR gate 711B, when either signal CEIO2U-00 or CEU2IO-00 is activated. Register 700 is enabled by the CESR2U-00 signal described infra and clocked by the CEI2SR-00 output signal from a decoder 357 in FIG. 4. Referring to FIG. 4, it will be noted that the signal CEU2I0 is applied to one input terminal of AND gate 354 and is AND&#39;ed with a strobe signal CTPHZD to generate the CEU2I0-10 signal at the output of AND gate 354. This signal is then applied to the enabling input of decoder 355. Also applied to input terminals 20A and 10A respectively of decoder 355 are bits 10 and 11 of the address 103 of the communication address unit. These bits are then decoded so as to activate one of four signals on the output terminals of decoder 355. When bits 10 and 11 on the input terminals 20 and 10 of decoder 355 are 1 and 0 respectively or decimal 2, then the output signal CEU2I0-A2 is enabled and is applied to the enable terminal of decoder 357. Additionally, bits 13 and 14 of the communication address unit (i.e., signals CADU13 and CADU14) are applied to input terminals 2PA and 1PA respectively of decoder 357. When both these bits 13 and 14 are zero, the zero output terminal of decoder 357 is enabled. Signal CEI2CN is activated through a NOR gate 358 and utilized to write into channel register 114 on FIG. 1A and 309 on FIG. 3. On the other hand, if bits 13 and 14 are 0 and  1 respectively and are applied as signals CADU13 and CADU14 to input terminals 2PA and 1PA respectively of decoder 357, then the 01 output terminal of decoder 357 is enabled and signal CEI2CB becomes active and is utilized to address CCB register 115 on FIG. 1A and CCB register 310 on FIG. 3. Finally, if bits 13 and 14 applied as signals CADU13 and CADU14 on input terminals 2PA and 1PA respectively of decoder 357 are 1 and 0 or decimal 2, then the 02 output terminal of decoder 357 becomes enabled and signal CEI2SR becomes active and is utilized to address the S register 123A. Thus signal CEU2I0 is utilized to enable the bus driver 111 and address registers 114, 115 and 123A. Accordingly, when the microprocessor 101 executes a write instruction directing it to write the microprocessor&#39;s accumulator into location A8 hexadecimal, the microprocessor places the contents of the microprocessor&#39;s accumulator on the U-bus, and enables bus driver 111 in a write direction which is then strobed into the appropriate register address. 
     When a write instruction is being executed and information is being written into any of the registers on the I-bus, bus driver 110 on FIG. 1A also enables the M-bus 109 and the same information written into the address register is also written into a section of memory 108 which is addressed. (See also FIGS. 2A and 2B). Bus driver 110 is enabled by the lack of signal CEMB2U and enables the M-bus in the direction of the memory 108. Hence the information written into the registers is also written into a &#34;shadow memory&#34; which preserves the information for diagnostic purposes or for debugging, and provides a place to preserve data when remote maintenance is performed. 
     In reading data from the I-bus to the U-bus it is necessary to inhibit transfers of data from the M-bus to the U-bus. This allows the I-bus to control the data onto the U-bus. This function is performed by generating the signal CEI02U in paging signal generator 105. This signal is then applied to the enable terminal of decoder 351 on FIG. 4, and at the same time bits 9 and 10 of the address 103 are applied to input terminals 2KA and 1KA respectively as signals CADU09 and CADU10. These signals undergo a first level of decode to provide an output signal CEI02U-A1 on output terminal 01 of decoder 351 when the input bits 9 and 10 are 0 and 1 respectively. The CEI02U-A1 signal is then applied to the enable input terminal of decoder 352 along with bits 14 and 15 on input terminals 2LA and 1LA respectively. Depending on the binary value of bits 14 and 15, 1 of 4 subcommand signals will be generated at the output terminals of encoder 352. When bits 14 and 15 have 0 values (which is equivalent to addressing location hexadecimal A8 in virtual memory space), a subcommand CEDH2I-00 is generated at the 00 output terminal of decoder 352. That signal is then applied to the enable terminal of register 360. Register 360 corresponds to the HI-order data register 120 of FIG. 1A. Accordingly, signal CEI02U provides a means for reading data from the HI-order data register 120 into the I-bus and onto the U-bus. However, since bus driver 110 has been inhibited by the presence of signal CEMB24, the memory space addressed by the address 103 is not read and only the high order register 120 is read. In a similar manner, low order data register 121 is read when bits 14 and 15 are 01 respectively, thus addressing output terminal 01 of decoder 352. Hence the signal CEDL2I-00 is generated which is applied to the enable terminal of register 361 on FIG. 4A. Thus, it is seen that registers 360 and 361 of FIG. 4A correspond to registers 120 and 121 respectively of FIG. 1A. 
     Bus driver 110 of FIG. 1A corresponds to driver 370 of FIG. 4B. This is a bidirectional driver and can drive data either from the memory bus 109 to the U-bus 112 or vice versa. The direction of data transfer is controlled by the signal CEMB2U. When the signal is asserted, data is permitted to flow from the memory bus 109 to the microprocessor bus 112. When it is not asserted, it permits data transfers in the other direction. The CEMB2U signal is generated via AND gates 371 and 372 of FIG. 4. These AND gates represent a simple AND&#39;ing operation of various signals such as the microprocessor read CUREAD, the strobe signal CTPHZ2 and the communication enable signals CESR2U and CEIN2U. They generate the CEMB2U signal which is then applied to one input terminal of AND gate 371. It should be noted when a transfer operation from the I-bus to the U-bus is being made, in other words a read operation from a register on the I-bus to the U-bus, then that signal would be high on the input of AND gate 371. If the remaining signals are also high the CEMB2U signal is high. When this high, signal is applied to the input terminal of driver 370 of FIG. 4B, it would inhibit information passing through via bus driver 110 from the I-bus to the M-bus. 
     Referring now to FIGS. 1A, 1C and 4, the addressing of USART&#39;s 116 and 117 will be disclosed. The signal that initiates this addressing procedure is CESU2U which appears as bit 3 from the paging signal generator 105 of FIG. 1A. It should be noted from FIG. 1C that the only address containing a 0 at bit number 3 is hexadecimal 88. Accordingly, when this location is addressed, bits 0, 3, 4 and 7 will become active. As noted previously, bit 3 provides the signal CESU2U which is applied to the enable terminal of decoder 350 of FIG. 3, and participates in the decoding of the middle bit of channel number CPGCNL. Depending on the value of that bit, one of two output signals CE0U2U and CE1U2U on output terminal 0 and 1 respectively of decoder 350 will become active. When CE0U2U is active, USART 16 is enabled whereas when CE1U2U is active USART 117 is enabled. When information is written on the enabled USART, it is also written into that portion of memory addressed by address 103. This type of dual addressing utilizes two virtual addresses which are converted into the same physical address to provide a duplicate address or shadow of the physical device actually being addressed. For example, we have shown how address 88 hexadecimal addresses the physical USART by providing an enable signal for the appropriate USART. Referring to FIG. 1C again, it will be noted that address C8 hexadecimal has the same binary map as address 88 hexadecimal with one exception: that is bit number 3 of address hexadecimal 88 is 0 or active, whereas bit 3 of address hexadecimal C8 is 1 or inactive. Accordingly, it is seen that with the exception of the third bit, address hexadecimal 88 provides the same signals as address hexadecimal C8 and thus C8 can be regarded as the shadow of the particular USART being addressed when the signal CE$U2U is provided at address hexadecimal 88. Hence it can be seen that address hexadecimal 88 or decimal 17 addresses the actual USART data whereas address hexadecimal C8 or decimal 25 addresses its shadow. Accordingly, when we write into address hexadecimal 88, it is written in the USART and the appropriate memory space addressed by memory space hexadecimal C8 in accordance with the principles discussed supra. Let&#39;s see how this happens by a typical example. First, we shall address paging signal generator 105 decimal location 17. Utilizing address format 103, the address will be as follows: 00000000000. It should be noted that bits 8 through 12 which do the actual addressing contain the decimal number 17 in binary format. Referring to FIG. 1C, it will be noted that decimal location 17 will have bits 0, 3, 4 and 7 active. Bit 0 generates signal CPGAD8 on paging signal generator 105, and this signal when active forces bit 7 to a 1. Bit 3 is the signal that addresses the USART and does not participate in changing the virtual address to a real address. Bit 4 of paging signal generator 105 is CPGAD4, and when this is active forces bits 4, 5 and 6 to a 1. Finally, bit 7 of paging signal generator 105 when active provides the signal CPGLIN and forces the high order bit and the middle order bit of the channel register into bit positions 8 and 9 of the real address. Assuming for the purposes of this example that those bit positions are 0, then bits 8 and 9 would be 0 and the final real address would be 000011110000000. Now when decimal address 25 is addressed, the address format 103 would take the form 0000000011001000. Again it will be noted from FIG. 1C that the following signals would be active: CPGAD8, CPGAD4 and CPGLIN. Since the signal CE$U2U which was also present in the previous example did not participate in changing the address, then final real address which would result is the same as the previous address given as follows: 000011110000000. Hence it is seen that the same address is addressed in memory. Accordingly, when remote maintenance or diagnostic operations are required, only the shadow memory may be addressed and not the actual USART. This same principle applies to other peripheral devices or registers attached to the I-bus also. 
     This shadow technique of addressing a piece of hardware such as the USART, the channel register or the channel control block register, etc. is particularly useful and saves cycle time when an interrupt occurs which requires a register such as the CCB register to have its contents replaced. In the conventional implementation, the contents of the register are read and stored in a temporary storage and the new information is written into the register. When it is required to replace the original contents of the register, the register is first read and the contents are stored in a temporary storage, and then the original contents are written back in. With the shadow concept, since there is a shadow of the original information of the register stored in a predetermined location in main memory, all that is required in an interrupt mode is for the new information to be written into the register. The old information will still be retained in the shadow location and when it is necessary to replace it back into the register, it can be read directly from the shadow location. Since it takes approximately 3 microseconds for a read and 4 microseconds for a write, a total of 4 microseconds is saved during each complete cycle. 
     Referring now to FIGS. 5A and 5B and FIGS. 6A and B, the preferred embodiment of the vectored interrupt invention will be disclosed. FIG. 6B shows the timing diagram of the interrupt sequence. The PTIME3 signal is a ground active TTL signal generated each 500 nanoseconds by the CPU 600 and has a nominal width of 100 nanoseconds. This signal is widely used by peripheral controllers of the system 600A to demark and set/reset conditions on buses 621, 622 and 623, and to act as data strobe. BINTR signal is the interrupt request signal which is an open collector driven line on the buses and is switched to ground by any peripheral controller of the system which is attempting to interrupt. The PIOCT signal is also a ground active TTL signal utilized by the CPU 600 to indicate to a selected controller that there are encoded states on the bus which convey bus dialog control information to the controller. The DATA BUS VALID SIGNAL indicates that the data on the bus is valid. The PBYTE signal when activated by the CPU 600 provides a controller such as the microprocessor 601 with the low order byte (i.e., bits 8-15) of the word being addressed. Finally, the PBBUSY or PROCED signal when active indicates that either a controller is busy and rejects the command or else it has accepted the last interrupt that was on the bus respectively. 
     The invention herein is concerned with the vectored interrupt, sometimes also referred to as a maskable interrupt. The μP interrupt mechanism herein described is activated by the CPU&#39;s (600) initiation of an I/O instruction addressed by channel number to one of the I/O devices (620 or 601) via the I/O bus. Should the order be directed to this μP (601), the decoder 601a1 (within 601A) recognizes the channel number and sets the μP interrupt request. The 16 bits of information on the system bus during command initiation (PIOCT see FIG. 6B) comprise the 10 bit channel number and a 6 bit Function Code. At the time the CN is compared, if it is determined to be the CN of this μP this information is stored in Register 362 (channel number) and 311 (FCN), and an interrupt, request to the μP is stored in a flip-flop 601a2 in 601A. Since the μP treats this as a &#34;masked interrupt&#34; its action may be deferred. Timing of these actions is shown in FIG. 6B. 
     Typically, when the microprocessor 601 honors an interrupt order, it sequentially addresses locations hexadecimal FFF8 and FFF9 in main memory 108 where the high order byte and low order byte respectively of the interrupt vector are stored. It will be seen that when the hexadecimal address FFF8 is placed in hexadecimal binary notation in the format denoted in FIG. 1A by numeral 103, it will have the following symbols: 1111111111111000. When hexadecimal notation FFF9 is placed in the above format, it will having the following symbols: 1111111111111001. Accordingly, when address FFF9 is addressed after the high order byte of the interrupt register is fetched, the 1 bits of the FFF9 address become active. These bit signals are applied in various combinations or individually to various hardware circuit elements shown on FIG. 3; in other words, these address bits instead of addressing hexadecimal address FFF9 in main memory 601b are used to (a) disable the paging address mechanism described supra and (b) to enable decoder 380 in FIG. 4c, which in turn (c) enables register 311 to provide a predetermined base address in PROM/RAM 601b, (d) which in turn contains the address of a selected one of 64 function codes. This is accomplished when bits 0, 1, 2, 13, 14 and 15 of the above address are applied to input terminals of decoder 380 of FIG. 4C. As noted supra, when address hexadecimal FFF9 is being addressed, bits 13 and 14 will be zero and bit 15 will be one, and these will be applied to the enable terminals of decoder 380. Moreover, bits 0, 1 and 2 of the address which will all be one&#39;s are applied to terminals 1, 2 and 4 of decoder 380 of FIG. 4C. Since the terminals on which bits 13 and 14 (i.e., signals CADU14 and CADU15) are applied have an inversion circuit decoder 380 which will be enabled, the information on terminals 1, 2 and 4 of decoder 380 will all be one&#39;s and will be decoded and provide an active signal CADUH7 on output terminal 7 of decoder 380. This is true because binary 111 decodes into decimal 7. The signal CADUH7 is then applied to the enable terminal F of register 311 of FIG. 3 and is further applied through an inverter 380A simultaneously to the enable/disable terminal of MUX&#39;s 301, 302, and 303 and AND gate 311A, thus disabling the paging address mechanism described supra. Accordingly, instead of accessing location hexadecimal FFF9, register 311 is enabled and provides the low order bits 10, 11, 12, 13, 14 and 15 at its output terminals. These bits form an address for addressing the RAM or PROM 601b. The contents of the address of memory 601b are placed in the low order byte position of the program counter (PC) 601d. The information in register 311 is the function code information shown on FIG. 5B which has been delivered by the CPU when an interrupt has been initiated (see FIG. 6B PIOCT signal). Since the paging address mechanism has been inhibited, and since signal CADUH7 redirects the addressing location hexadecimal FFF9 to register 311, and since signal CADUH7 additionally enables register 311, the low order bits 10, 11, 12, 13, 14 and 15 of register 311 are used to access information in memory 601b which is placed in the low order byte position of program counter (PC) 601d. Since there are 6 bits in the low order byte of the function code, it can address up to 64 locations in memory such as the RAM or PROM 601b. Each of these locations contains the low order byte of the interrupt vector associated with its function code, which when appended comprises the interrupt address. 
     Accordingly, depending on the information content of the function code, there is provided an interrupt which can start at any of 64 different locations in the PROM 601b and thus can provide up to 64 different interrupt operations. Moreover, as seen supra, the program computer counter 601d or other registers obtain their information from a shadow location. Thus, it is not necessary to first unload and then save the old information, and then load the new information and repeat the same process over again when going out of the interrupt loop. It should also be noted that the normal interrupt procedure of the microprocessor which directs the micro-processor to address FFF9 on interrupts is bypassed; and one address of 64 unique addresses is automatically provided by the function code, thus providing in effect a shortcut to 64 different vectors or routines. 
     Referring now to FIGS. 4 and 6B, it should be noted that when signal CADUH7 is activated, it is applied along with a phase D signal (CTPHZD) to AND gate 358A enabling signal CEI2CN. Moreover, signal CADUH7 or OR&#39;ed with signal CECN2I 1H OR gate 358B to activate signal CECN2I. Signal CEI2CN is applied to the C terminal of channel register 309 on FIG. 3, which as has been previously discussed, is equivalent to channel register 114A on FIG. 1A. Similarly, signal CECN2I is used for strobing the contents of channel number register 122 on FIG. 1A onto the I-bus or vice versa. 
     Accordingly, the vector interrupt does two things: (a) it automatically addresses any one of 64 locations in accordance to a unique function code; and (b) it loads the hardware channel number register to the channel number that we are being asked to service on the interrupt. The channel control block register (CCB) 115 may also be changed subsequently by firmware on an interrupt depending on what the function code is. 
     Having shown and described a preferred embodiment of the invention, those skilled in the art will realize that many variations and modifications may be made to affect the described invention and still be within the scope of the claimed invention. Thus, many of the elements indicated above may be altered or replaced by different elements which will provide the same results and fall within the spirit of the claimed invention. It is the intention, therefore, to limit the invention only as indicated by the scope of the claims.