Re: Writing .cpu file for Z80

["Frank Ch. Eigler" <fche@redhat.com>]

Hi -

> The Z80 CPU uses prefix opcodes to change opcode tables and operation
> registers. For example, <0x21 0x34 0x12> is "LD HL,0x1234", but <0xdd 0x21
> 0x34 0x12> is "LD IX,0x1234". Do I need write both instructions or is it
> possible to write rule which selects proper instruction set or extract
> correct register index?

Consider writing prefix opcodes as a normal part of the instruction
(so include them in the base-insn-bitsize).  So you'd need two
separate instructions, one with and one without.  You may be able to
use cgen macros to generate both flavours from one piece of
declaration.


> Z80 has special rule to form indirect memory operations using index
> registers. There immediate offset is always third byte of instruction. For
> example: <0xdd 0x34 0x12> is "INC (IX+0x12)", <0xfd 0x36 0xfe 0xab> is "LD
> (IY-0x02),0xAB"... But there are "strange" instructions, where index is
> placed before opcode itself: <0xdd 0xcb 0x10 0x1e> is "RR (IX+0x10)" (<0xcb
> 0x1e> is "RR (HL)", <0xcb> is opcode prefix, which select another opcode
> table) and undocumented one <0xdd 0xcb 0x10 0x1f> "RR (IX+0x10),A". Should
> I write 2 instructions types?

Probably, if the same operands are not in the same place.


> eZ80 uses four opcode prefixes (.SIS, .SIL, .LIS, .LIL) which set operation
> mode (.IL - long instruction (24 bit immediate), .IS - short instruction
> (16 bit immediate), .S - 16 bit processing, .L - 24 bit processing): <0x40
> 0x21 0x34 0x12> is "LD.SIS HL,0x1234" and <0x5b 0x21 0x56 0x34 0x12> is
> "LD.LIL HL,0x123456".  These prefixes can be applied to all instructions
> (but it has no sense for part of instructions). Moreover, assembler should
> support short mode of instructions (.S, .L, .IS, .IL), which is completed
> by assembler depending on compiling mode (ADL or Z80). Should I generate
> all possible combinations (9 x instruction_set)? Is there more correct
> solution?

You may be able to represent tehse opcode prefix bytes as an operand,
and have a special asm parser/printer that sets them from the .SIS etc
mnemonic suffix.  Or you can go brute-force and generate the family of
9 (!) instructions with a cgen macro.  (There may be other ways too.)

- FChE