func (p *Process) ReadableN(a Address, n int64) bool {
	for {
		m := p.findMapping(a)
		if m == nil || m.perm&Read == 0 {
			return false
		}
		c := m.max.Sub(a)
		if n <= c {
			return true
		}
		n -= c
		a = a.Add(c)
	}
}
func (p *Process) ReadableN(a Address, n int64) bool {
	for {
		m := p.findMapping(a)
		if m == nil || m.perm&Read == 0 {
			return false
		}
		c := m.max.Sub(a)
		if n <= c {
			return true
		}
		n -= c
		a = a.Add(c)
	}
}
// ReadableN reports whether the n bytes starting at address a are readable.