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From: John Savard <quadibloc@servername.invalid>
Newsgroups: comp.arch
Subject: Re: "Mini" tags to reduce the number of op codes
Date: Mon, 08 Apr 2024 07:05:35 -0600
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On Sun, 7 Apr 2024 20:41:45 +0000, mitchalsup@aol.com (MitchAlsup1)
wrote:

>How do you attach 32-bit or 64-bit constants to 28-bit instructions ??

Yes, that's a problem. Presumably, I would have to do without
immediates.

An option would be to reserve some 16-bit codes to indicate a block
consisting of one 28-bit instruction and seven 32-bit instructions,
but that means a third instruction set.

>How do you switch from 64-bit to Byte to 32-bit to 16-bit in one
>set of 256-bit instruction decodes ??

By using 36-bit instructions instead of 28-bit instructions.

>In complicated if-then-else codes (and switches) I often see one inst-
>ruction followed by a branch to a common point. Does your encoding deal 
>with these efficiently ?? That is:: what happens when you jump to the
>middle of a block of 36-bit instructions ??

Well, when the computer fetches a 256-bit block of code, the first
four bits indicates whether it is composed of 36-bit instructions or
28-bit instructions. So the computer knows where the instructions are;
and thus a convention can be applied, such as addressing each 36-bit
instruction by the addresses of the first seven 32-bit positions in
the block.

In the case of 28-bit instructions, the first eight correspond to the
32-bit positions, the ninth corresponds to the last 16 bits of the
block.

John Savard