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From: mitchalsup@aol.com (MitchAlsup1)
Newsgroups: comp.arch
Subject: Re: Why VAX Was the Ultimate CISC and Not RISC
Date: Tue, 11 Mar 2025 00:53:31 +0000
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On Mon, 10 Mar 2025 22:40:55 +0000, BGB wrote:

> On 3/7/2025 9:28 PM, MitchAlsup1 wrote:
>> On Sat, 8 Mar 2025 2:49:50 +0000, BGB wrote:
>>
>> ------------------------
>>>
>>> I guess, while a person could do something like (in C):
>>>    _BitInt(1048576) bmp;
>>>    _Boolean b;
>>>    ...
>>>    b=(bmp>>i)&1;  //*blarg* (shift here would be absurdly expensive)
>>>
>>> This is liklely to be rare vs more traditional strategies, say:
>>>    uint64_t *bmp;
>>>    int b, i;
>>>    ...
>>>    b=(bmp[i>>6]>>(i&63))&1;
>>
>> Question: How do you handle the case where the bit vector is an odd
>> number of bits in width ?? Say <3, 5, 7, 17, ...>
>>
>
> It is rare for bitmap bits to not be a power of 2...
>
> I would guess, at least for C, something like (for 3 bits):
>    uint32_t *bmp;
>    uint64_t bv;
>    int i, b, bp;
>    ...
>    bp=i*3;
>    bv=*(uint64_t *)(bmp+(bp>>5));
>    b=(bv>>(bp&31))&7;
>
> Could apply to anything up to 31 bits.

Not bad.

> Could do similar with __int128 (or uint128_t), which extends it up to 63
> bits.
------------
>> Mc 68020 had instructions to access bit-fields that cross word
>> boundaries.
>>
>
> I guess one could argue the use-case for adding a generic funnel shift
> instruction.

My 66000 has CARRY-SL/SR which performs a double wide operand shifted
by a single wide count (0..63) and produces a double wide result {IO}.

> If I added it, it would probably be a 64-bit encoding (generally needed
> for 4R).

By placing the width in position {31..37} you can compress this down
to 3-Operand.

----------
>> Architecture is more about what gets left OUT than what gets left IN.
>
> Well, except in this case it was more a question of trying to fit it in
> with C semantics (and not consideration for more ISA features).

Clearly, you want to support C semantics--but you can do this in a way
that also supports languages with real bit-field support.
---------------
> There are still some limitations, for example:
> In my current implementation, CSR's are very limited (may only be used
> to load and store CSRs; not do RMW operations on CSRs).

<y 66000 only has 8 CPU CRs, and even these are R/W through MMI/O
space. All the other (effective) CRs are auto loaded in line quanta.

This mechanism allows one CPU to figure out what another CPU is up to
simply by meandering through its CRs...

> Though, have noted that seemingly some number of actual RISC-V cores
> also have this limitation.
>
>
> A more drastic option might be to try to rework the hardware interfaces
> and memory map hopefully enough to try to make it possible to run an OS
> like Linux, but there doesn't really seem to be a standardized set of
> hardware interfaces or memory map defined.
>
> Some amount of SOC's though seem to use a map like:
>    00000000..0000FFFF: ROM goes here.
>    00010000..0XXXXXXX: RAM goes here.
>    ZXXXXXXX..FFFFFFFF: Hardware / MMIO

My 66000::
  00 0000000000000000..FFFFFFFFFFFFFFFF: DRAM
  01 0000000000000000..FFFFFFFFFFFFFFFF: MMI/O
  10 0000000000000000..FFFFFFFFFFFFFFFF: config
  11 0000000000000000..FFFFFFFFFFFFFFFF: ROM

Whatever you are trying to do, you won't run out of address space until
64 bits becomes insufficient. Note: all HW interfaces are in config
space
and all CRs are in MMI/O space.

------------
> They seem to also be asking for a UEFI based boot process, but this
> would require having a bigger BootROM (can't likely fit a UEFI
> implementation into 32K). Seems that the idea is to have the UEFI BIOS
> boot the kernel directly as an ELF image (traditionally UEFI was always
> PE/COFF based?...).

Boot ROM should be big enough that no BOOT ROM will ever exceed its
size.
--------------
> There is a probable need to move away from the "BJX2" name, which as
> noted, has some unfortunate connotations (turns out it was also used for
> a lewd act) and seems to be triggering to Google's automatic content
> filtering (probably for a similar reason).

Hilarious--and reason enough to change names.

When you do change names, can you spell LD and ST instead of MOV ??