Re: [colorforth] Understanding the x18 instruction jump table
- Subject: Re: [colorforth] Understanding the x18 instruction jump table
- From: "Helen Austin" <gwenhwyfaer@xxxxxxxxx>
- Date: Tue, 13 Jun 2006 23:08:30 +0100
On 6/13/06, John Drake <jmdrake_98@xxxxxxxxx> wrote:
: inst ( n ) 1 slot +! 1f and jump jmp jmp call call
jz jz jns jns @b @+ n @a !b !+ nul !a -x 2*x 2/x +*
orx andx nul +x r@ a@ t@ s@ r! a!x nul t!
Why are the opcodes jmp, call and jns repeated?
At a guess, because the LSB of those four instructions (call, jmp, jz,
jns) either affects their operation or is ignored when decoding them.
Note that after "1f and jump" there are exactly 32 words - one for
every 5-bit opcode, I'd guess.
Helpful?
Regards
Helen
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