By: Megol (golem960.delete@this.gmail.com), August 12, 2022 2:04 am
Room: Moderated Discussions
Brett (ggtgp.delete@this.yahoo.com) on August 11, 2022 3:26 pm wrote:
> Any 68k manual, each instruction spells out the bits:
> https://web.njit.edu/~rosensta/classes/architecture/252software/code.pdf
>
> There is unused state in the mode bits that can be used to add another 8 data registers transparently for
> one source.
FOR A SMALL SUBSET OF OPERATIONS!
That's the thing - it's not something that's really available as an extension, it's a very lopsided hack.
>Though instead the AMMX extension added 32 bit instructions and used a similar format.
It also wasted the only reasonable opcode space available for a prefix (11 free bits) to add MMX support.
At least happy they moved from supporting the mostly useless ColdFire sign extension opcodes to use the encoding space as a prefix operation as I had been proposing.
Of course my prefix ideas were more general.
> ColdFire obsoleted the 2 of the 3 opmode operation size bits for all but MOVE loads/stores. That frees up
> huge opportunities for expansion. The plan may have been to use these bits to double the register file.
Not relevant as the 68080 is intended to be compatible with Amiga software.
> Ugly, but not so ugly as x86.
Debatable. X86 is like two separate architectures with some similarities, the 8086-80286 was pretty clean, the 80386 too.
> Any 68k manual, each instruction spells out the bits:
> https://web.njit.edu/~rosensta/classes/architecture/252software/code.pdf
>
> There is unused state in the mode bits that can be used to add another 8 data registers transparently for
> one source.
FOR A SMALL SUBSET OF OPERATIONS!
That's the thing - it's not something that's really available as an extension, it's a very lopsided hack.
>Though instead the AMMX extension added 32 bit instructions and used a similar format.
It also wasted the only reasonable opcode space available for a prefix (11 free bits) to add MMX support.
At least happy they moved from supporting the mostly useless ColdFire sign extension opcodes to use the encoding space as a prefix operation as I had been proposing.
Of course my prefix ideas were more general.
> ColdFire obsoleted the 2 of the 3 opmode operation size bits for all but MOVE loads/stores. That frees up
> huge opportunities for expansion. The plan may have been to use these bits to double the register file.
Not relevant as the 68080 is intended to be compatible with Amiga software.
> Ugly, but not so ugly as x86.
Debatable. X86 is like two separate architectures with some similarities, the 8086-80286 was pretty clean, the 80386 too.
