CPU Description
Break for Emulation (BRKEM)
This is the basic instruction used to start the 8080 emulation mode.
This instruction operates identically to the software interrupt
instruction BRK [INT], except it resets the mode flag MD to 0 in the
PSW [FL]. The PSW, PS, and PC [FL, CS, and IP] registers are
pushed onto the stack. The MD flag is then reset, and the interrupt
vector specified by the immediate operand in this instruction is loaded
into the PS and PC [CS and IP]. This effectively performs a jump to
an interrupt service routine written in 8080 code.
It is prohibited to nest BRKEM instructions. In other words, the
routine must finish with a return from emulation before another
emulation call may be made using BRKEM. If an interrupt occurs
during the emulation mode, the V20 is forced back to native mode
and native mode is used for this interrupt service routine (ISR). A call
to emulation mode may not take place within this ISR.
Return From Emulation (RETEM)
When in 8080 emulation mode, RETEM may be used to return to
native mode. The V20 restores the PSW, PS, and PC [FL, CS, and IP]
registers from the stack as it would when returning from an interrupt
service routine. At the same time, the contents of the MD flag are
restored to a logical 1 since the flag register (PSW) is popped from
the stack.
6-14
Содержание ZT 8809A
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Страница 334: ...Jumper Configurations W49 W48 A W47 B A B W37 W66 A W50 B C J7 W67 Figure A 11 W37 W47 50 W66 W67 Jumper Blocks A 34...
Страница 346: ...Jumper Configurations W61 W60 W62 W63 W64 A B A B W65 Figure A 13 W60 W65 Jumper Block A 46...