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User’s Manual 259
20. DIFFERENCES RABBIT VS. Z80/Z180
INSTRUCTIONS
The Rabbit is highly code compatible with the Z80 and Z180, and it is easy to port non I/O
dependent code. The main areas of incompatibility are instructions that are concerned with
I/O or particular hardware implementations. The more important instructions that were
dropped from the Z80/Z180 are automatically simulated by an instruction sequence in the
Dynamic C assembler. A few fairly useless instructions have been dropped and cannot be
easily simulated. Code using these instructions should be rewritten.
The following Z80/Z180 instructions have been dropped and there are no exact substi-
tutes.
DAA, HALT, DI, EI, IM 0, IM 1, IM 2, OUT, IN, OUT0, IN0, SLP, OUTI,
IND, OUTD, INIR, OTIR, INDR, OTDR, TESTIO, MLT SP, RRD, RLD, CPI,
CPIR, CPD, CPDR
Most of these op codes deal with I/O devices and thus do not represent transportable code.
The only opcodes that are not processor I/O related are MLT SP, DAA, RRD, RLD, CPI,
CPIR, CPD, and CPDR. MLT SP is not a practical op code. The codes that are concerned
with decimal arithmetic, DAA, RRD, and RLD, could be simulated, but the simulation is very
inefficient. (The bit in the status register used for half carry is available and can be set and
cleared using the PUSH AF and POP AF instructions to gain access.) Usually code that
uses these instructions should be rewritten. The instructions CPI, CPIR, CPD, and CPDR
are repeating compare instructions. These instructions are not very useful because the scan
stops when equal compare is detected. Unequal compare would be more useful. They are
difficult to simulate efficiently, so it is suggested that code using these instructions be
rewritten, which in most cases should be quite easy.
The following op codes are dropped.
RST 0, RST 8, RST 0x30
The remaining RST instructions are kept, but the interrupt vector is relocated to a variable
location the base of which is established by the EIR register. RST can be simulated by a
call instruction, but this is not done automatically by the assembler since most of these
instructions are used for debugging by Dynamic C.
The following instruction has had its op code changed.
EX (SP),HL - old opcode 0x0E3, new opcode - 0x0ED-0x054