Re: Why VAX Was the Ultimate CISC and Not RISC

On 2025-03-10 8:53 p.m., MitchAlsup1 wrote:

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:
>
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>
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

 

I got used to a lot of micro-computer maps which have RAM at low end and ROM at the high end.
I have the Q+ SoC setup like this (which is pretty much the way I setup all SoC) I use 32-bit addressing for demo purposes. Low cost FPGA boards typically do not have many gigabytes of memory.
00000000..3FFFFFFF DRAM
40000000..CFFFFFFF DRAM expansion area (not present on board)
D0000000..DFFFFFFF config (FTA DDBB device discovery black boxes)
E0000000..FEBFFFFF unassigned
FEC00000..FEFFFFFF MMI/O
FF000000..FFF7FFFF ROM expansion area (unimplemented)
FFF80000..FFFBFFFF      Scratchpad RAM
FFFC0000..FFFFFFFF      ROM
The DDBB can configure the MMI/O to appear at any address, but I have devices defaulting to
FEC00000..FEFFFFFF MMI/O
Each DDBB is a 4kB space, the last 3kB are ROM for device support
There is a CSR that can reconfig the config area, which defaults to Dxxxxxxx.
Addresses are double bit sign extended from 32-bits to 64-bits when setup by the boot ROM, but the high order 64-bits are ignored by the hardware.
I went with dynamically determined MMI/O addressing setup at boot time which is hopefully usable with and OS. Defaults are such that the system is able to boot.

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

How does one reference DRAM vs MMI/O at the same address using a LD / ST instruction?
Q+ CPU just uses a 64-bit address range. The config space is specified in a CR defaulting to FFFFFFFFDxxxxxxx The TLB is setup at boot to access ROM at FFFFFFFFFFFCxxxxx Otherwise there is no distinction with addresses. There is a region table in the system that describes up to eight distinct regions.

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.

Are there any CRs accessible with any instructions besides LD / ST?

 ------------

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.
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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).

Coming up with names is surprisingly difficult. I got into a discussion with a colleague a while ago about this. They were having difficulty coding something an it turned out to be simply what names to choose for routines.

 Hilarious--and reason enough to change names.
 When you do change names, can you spell LD and ST instead of MOV ??

Yes, please LD / ST it is so much clearer what is going on. Less trouble getting confused by the placement of operands.
I always put the memory operand second, which breaks the pattern of having the destination operand first. Otherwise the destination is first.
I go cross-eyed reading code that is a whole lot of moves.