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Zilog Z8000
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===Registers=== There are sixteen 16-bit [[Processor register|registers]], labeled R0 through R15. The registers can be concatenated into eight 32-bit registers, labeled RR0/RR2/../RR14, or into four 64-bit registers, labeled RQ0/RQ4/RQ8/RQ12. The first eight registers can be also subdivided into sixteen 8-bit registers, labeled RL0 though RL7 for the lower byte and RH0 through RH7 for the upper (high) byte. Register R15 is designated as [[stack (data structure)|stack]] [[pointer (computer programming)|pointer]]. On the Z8001, register R14 is used to include a fixed segment in the stack pointer, and the [[program counter]] is expanded to 32 bits to include a similar segment. There is both a [[user mode]] ("normal") and a [[supervisor mode]], selected by bit 14 in the flag register. In supervisor mode, the stack registers point to the system stack and all privileged instructions are available. In user mode, the stack registers point to the normal stack and all privileged instructions will generate a fault. Having separate modes and stacks greatly adds to the performance of [[context switch]]es between user programs and an [[operating system]].<ref name="abramovitz1981">{{cite book | first1=Bob | last1=Abramovitz | first2=Janice | last2=Enger | first3=Curtis | last3=Ingraham | first4=Susanna | last4=Jacobson | first5=Patrick | last5=McGuire | title=Osborne 16-Bit Microprocessor Handbook | date=1981 | isbn=0-931988-43-8 | publisher=Osborne/McGraw-Hill }}</ref>{{rp|pages=6.1}} [[File:Zilog_Z8002_die.JPG|right|180px|thumb|[[Die (integrated circuit)|Die]] of Zilog Z8002]]
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