Specifications

Processor Description (V40)
8080 EMULATION
Designs based on 8080 and 8085 microprocessors have two major
limitations: inadequate performance and lack of development tools.
Upgrading an 8-bit design to a higher performance microprocessor
requires time to convert the software. The V40 solves these problems
by supporting two modes of operation: emulation mode and native
mode. When the CPU is in emulation mode, it executes the 8080
instruction set. Emulation mode is used for the existing software base.
In native mode, the CPU executes the 8088 instruction set. All future
software development is done in native mode to take advantage of the
8088 instruction set and the large number of development tools
designed around it.
The CPU powers up in native mode, the normal mode of operation.
Two instructions are provided to switch the CPU from native mode to
emulation mode and back. Break for Emulation (BRKEM) is the
instruction used to switch from native to emulation mode; Return
from Emulation (RETEM) is used to switch back. The effect of these
instructions and emulation mode operation is discussed below.
The BRKEM instruction is similar to the BRK [INT] software
interrupt. BRKEM includes an 8-bit vector that, when multiplied by
four, points to the location in the interrupt vector table that contains
the address of the 8080-based routine. During execution of this
instruction, the CPU saves the machine status by pushing the current
contents of the PSW [FL] and the return address onto the stack. The
CPU then clears the mode (MD) flag to a logical 0 and loads the
address of the emulation mode routine into the PS [CS] and PFP [IP].
The RETEM instruction is one of the four methods of terminating
emulation mode. The execution of RETEM is identical in operation to
the RETI [IRET] instruction. Upon executing this instruction, the
CPU restores the contents of the PSW [FL], PS [CS], and PFP [IP]
from the stack, returning program execution to the instruction
following BRKEM. The other three methods of exiting emulation
mode are a system reset, a hardware interrupt, or the CALLN
instruction.
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