fig1 illustrates a logical structure of a computer system operating according to the present invention . the real instruction processing units ( cpus ) and main storage are shown at 100 . the one or more processing units are operated by a host operating system 102 . the host operating system ( vm / xa ) of the present invention provides an ability to partition the real processor and storage resources into virtual machines . the host operating system of the preferred embodiment is the ibm vm / xa system product . ( vm / xa is a trademark of the ibm corporation ). the host system allows the creation of level 1 virtual machines shown generally at 104 , referred to as first level guests . each of these guests 106 , 108 , 110 and 112 , is controlled by an operating system which operates as though it was an independent machine . if the first level guest operating system is capable of subdividing its virtual machine into an additional level of virtual machines , level 2 virtual machines 114 may be created . thus , for example , if the first level guest operating system is the ibm vm / xa system product 106 , it may in turn create level 2 virtual machines ( second level guests ) operating , for example , the ibm conversational monitor system ( cms ) as shown at 116 , 118 , 120 and 122 . the creation of first level and second level guests occurs through the execution of a start interpretive execution ( sie ) instruction . the sie instruction establishes the environment necessary for the operation of the virtual machine . the initiation of a first level guest by the sie instruction causes host values in certain machine facilities such as control registers , program status word , and general registers 14 and 15 , to be saved in reserved working storage and at locations in the system area of the main storage , referred to as the host save area . guest values from the guest . state description control block are then loaded into those machine facilities and guest execution begun . each guest uses the system facilities on a time slice basis . an attempt by the first level guest to initiate a guest of its own ( a second level guest ) would , in the prior art , result in control being passed back to the host programming system which would then run the second level guest as a first level guest but at degraded performance . the attempt by the first level guest to initiate a second level guest in the prior art resulted in an instruction interception . the intercepted instruction processing logic caused the first level guest to exit , the second level guest to be initiated as a first level guest , and then , upon completion of second level guest initiation , a return to the first level guest . this interception processing introduced a large amount of overhead for second level guest execution resulting in substantially degraded performance . the computer system hardware in the prior art was not able to interpret second level guest because hardware mechanisms were provided only to handle a first level guest . in addition , a pageable second level guest whose dynamic address translation ( dat ) function required both guest and host translation , was required to maintain both guest and host translation parameters in the hardware . the addition of the second level guest therefore required a three level address translation , further degrading performance . the present invention is directed to eliminating these problems with second level guest execution and improving system performance . the present invention is directed to providing the apparatus and method to allow interpretive execution of second level guest instructions . in the preferred embodiment of the present invention , initiation of a first level guest occurs in the same manner as in the prior art . fig2 illustrates guest initiation according to the present invention . the machine facilities , including psw , control registers and general registers , are shown generally at 210 . initiation of a first level guest through an sie instruction causes machine facilities associated with the host to be stored in the host save area 212 . first level guest machine facility information is loaded from state description 214 ( sd1 ) to become modified machine facilities 210 &# 39 ;. the present invention allocates a new fixed storage area designated as the first level guest save area . first level guest save area 216 is available for storage of first level guest machine facilities 210 &# 39 ; upon execution of an sie instruction creating a second level guest . after storage of first level guest facilities , second level guest facility information is loaded from state description 218 ( sd2 ) into machine facilities becoming 210 &# 39 ;&# 39 ;. the machine facility areas 210 , 210 &# 39 ; and 210 &# 39 ;&# 39 ; represent the real machine facilities as they change over time due to loading the mode field da % a for the appropriate level of guest execution . an exit from the second level guest causes machine facility information 210 &# 39 ;&# 39 ; to be stored back into state description 218 and first level machine facility information 210 &# 39 ; to be reloaded from first level guest save area 216 . an exit from the first level guest causes first level guest facility information to be stored back to state description 1 214 and host save area information 212 to be reloaded into machine facilities 210 . the preferred embodiment of the present invention provides the apparatus and methods necessary to divide real storage into fixed regions for each first level guest virtual machine . this facility is known as the region relocate facility ( rrf ). the region relocate facility allows guest absolute addresses to be mapped directly into main storage addresses by means of a zone origin and zone extent . the zone origin ( zo ) is a real memory address defining the beginning of storage allocated for a particular first level guest machine . as illustrated in fig3 zone origin 310 indicates the starting point for storage allocated to first level guest # 2 312 . a second value labelled zone extent ( ze ) 314 defines the extent of the memory region allocated to the first level guest . similar regions 316 and 318 are defined for the other first level guests shown in fig3 . the example of three first level guests as shown in fig3 is by way of illustration only ; the apparatus and methods of the present invention are not limited to three first level virtual machines . the dynamic address translation control array shown in fig4 c maintains region information for each first level guest including zone origin 430 and zone extent 432 . memory area 320 is designated as the host system area and is used to maintain the various data and programs needed by the host processing system . state descriptions for first level guest systems are stored in the host system area 320 . the state description for virtual machine # 1 is shown at 330 and for the other two example machines at 332 and 334 . the state descriptions for second level virtual machines are stored in the memory region allocated to the initiating first level virtual machine . thus , for example , second level state description 336 is stored within virtual machine # 2 region of memory 312 . the preferred embodiment is implemented through the provision of microcode added to the start interpretive execution instruction . this microcode causes the saving and loading of the machine facilities and includes the definition for first level guest save area 216 . the microcode uses reserved working store arrays in the execution element of the central processor and system area locations to save various host and first level guest values as shown in fig4 a and 4b . the allocation of fixed regions of memory for each first level guest also simplifies the address translation for pageable second level guests . instead of requiring three levels of translation as the prior art required , the final level of host translation is replaced by a simple adding of a zone origin that relocates the first level guest into its own zone or partition in storage . a dynamic address translation control array stores the zone definition information for the guests as shown in fig4 c . the overall translation process for a pageable second level guest involves the use of second level and first level guest control registers ( for address translation ) as well as the zone origin . in the preferred embodiment , a special mode indicator is set in the hardware whenever a second level guest is operating so that the hardware will know when to employ enhanced address translation for a second level guest . the provision of a first level guest save area and the logic for saving and restoring first level facilities and the provision of additional translation parameter facilities with the appropriate operational control allow the hardware and microcode to interpretively execute a second level guest using native machine facilities without exiting the first level guest and incurring the overhead necessary to monitor that exit and reinitiation . in the preferred embodiment of the present invention , apparatus and methods directed at providing second level guest timing facilities are implemented . the ibm 3090 computer systems on which the present invention has been implemented , provide only a single set of hardware implemented timing facilities . these include a time - of - day ( tod ) clock , a cpu timer and clock comparator . the prior art systems only provided for first level guest timing . the first level guest timing facilities were implemented without adding additional hardware ( i . e ., without a second hardware clock ) by providing microcode using the clock comparator hardware to maintain a difference or delta value between the host clock and first level guest clock ( see fig5 ). this implementation generates an interrupt at the closest impending interrupt whenever : 1 ) the host clock comparator 510 value is less than the host tod ; 2 ) the guest clock comparator value is less than the guest tod ( the guest tod is equal to the host tod plus a delta for the epoch value 514 specified by the host program ); 3 ) the guest cpu timer value is less than zero . the actual values related to the host clock comparator , the guest clock comparator and the guest cpu timer are kept in a first level guest local reserved working store array as shown in fig5 . the enablement of direct second level guest interpretive execution requires a change to the timing facilities . the prior art systems initiated second level guests as first level guests and could therefore use the first level guest timing facilities . direct interpretation of second level guests requires an associated second level guest timing facility . the second level guest timing facilities are constrained in that both host timing facilities and first level guest timing facilities must continue to operate while the second level guest is operating . thus , an interrupt caused by the host timer or first level guest timer must cause a real processor interrupt . the second level guest clock comparator and cpu timer values are kept in second level guest local reserved working store arrays ( fig6 ). constant monitoring of timing facilities for the second level guest , the first level guest and the host would add overhead and reduce processing efficiency . in the preferred embodiment of the present invention , the second level timing facilities are only checked periodically and not at every clock tick . the periodic checking , in the preferred embodiment , coincides with hardware signalled processing of pending interrupt conditions . this processing occurs , for example , every 256 microseconds and includes an evaluation of the second level guest clock comparator 612 and cpu timer values relative to the hardware tod value to determine whether an interrupt should be taken . the second level guest timing facilities are checked only when a second level guest is running . the timing facility values are stored in a local reserve working store array as shown in fig6 . the difference between the second level guest tod and the host tod ( referred to as the delta or epoch of the second level ) is , in the preferred embodiment , the sum of the first level guest epoch and the second level guest epoch and is stored in the working store array 610 . the second level guest timing facility analysis performs two tests to determine whether a second level guest timing interrupt has become pending . first , the guest clock comparator , adjusted by the sum of the epochs , is subtracted from the current host tod . if the result is positive ( tod exceeds clock comparator ), then a second level guest clock comparator interrupt is pending . if the appropriate masking ( guest # 2 psw bit 7 and guest # 2 control reg 0 bit 20 ) is enabled an external interrupt is taken by the second level guest . if either mask is disabled , the second level guest clock comparator interrupt pending flag is set in the guest # 2 timing flag area 614 . second , the guest # 2 cpu timer , adjusted by the value of the host tod at the time when the guest # 2 cpu timer was loaded is subtracted from the current host tod . if the result is positive ( guest # 2 cpu timer less than 0 ), then a guest # 2 cpu timer interrupt is pending . if the appropriate masking is enabled , an external interrupt is taken by the second level guest . if either mask is disabled , the guest # 2 cpu timer interrupt pending flag is set in the guest # 2 timing flags 614 . the interrupt pending flags will be checked each time a new psw or control reg 0 is loaded to determine whether a pending interrupt is now enabled and should be taken . the approach of the preferred embodiment allows implementation of second level guest timing facilities without requiring additional clock hardware . this implementation provides for timing facilities at the second level without a decrease in performance . the preferred embodiment has been described in terms of a microcode implementation . it will be understood by those in the art that these functions could also be implemented directly in hardware in a combination of hardware and microcode . it will be understood from the foregoing description that various modifications and changes may be made in the preferred embodiment of the present invention without departing from its true spirit . it is intended that this description is for purposes of illustration only and should not be construed in a limiting sense . the scope of this invention should be limited only by the language of the following claims .