CPU Description
MODE OPERATIONS - 8080 EMULATION MODE
Designs based on 8080 and 8085 microprocessors have two major
limitations: not enough performance and lack of development tools.
Upgrading an 8-bit design to a higher performance microprocessor
requires time to convert the software. The V20 solves these problems
by supporting two modes of operation: emulation and native.
When the CPU is in native mode, it executes the 8088/8086 family of
instructions along with the V20 enhanced and unique instructions.
When
in
emulation
mode,
the
CPU
emulates
the
8080
microprocessor. All future software development is done in native
mode to take advantage of the V20 instruction set and the larger
number of software development tools available.
The V20 processor powers up in native mode (the normal mode of
operation). The mode flag in the PSW [FL] register, bit 15, indicates
operation in the native mode when set to 1 and emulation mode when
set to 0. This bit is set and reset, both directly and indirectly, by
executing the mode manipulation instructions.
Two of these instructions are provided to switch operation from the
native mode to the emulation mode and back: Break for Emulation
(BRKEM) and Return From Emulation (RETEM). The other two
instructions are used to switch from the emulation mode to the native
mode and back: Call Native Routine (CALLN) and Return From
Interrupt (RETI). The system automatically returns from the 8080
emulation mode to the native mode when the RESET signal is
present, or when an external interrupt is present. The type of interrupt
may be maskable or non-maskable; interrupts are described more
fully in Chapter 12, "Interrupt Controller."
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