Re: Why VAX Was the Ultimate CISC and Not RISC

On Sat, 1 Mar 2025 22:29:27 +0000, BGB wrote:

On 3/1/2025 5:58 AM, Anton Ertl wrote:

Lawrence D'Oliveiro <ldo@nz.invalid> writes:

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Would likely need some new internal operators to deal with bit-array
operations and similar, with bit-ranges allowed as a pseudo-value type
(may exist in constant expressions but will not necessarily exist as an
actual value type at runtime).
   Say:
     val[63:32]
Has the (63:32) as a BitRange type, which then has special semantics
when used as an array index on an integer type, ...

Mc 88K and My 66000 both have bit-vector operations.

The previous idea for bitfield extract/insert had turned into a
composite BITMOV instruction that could potentially do both operations
in a single instruction (along with moving a bitfield directly between
two instructions).

Using CARRY and extract + insert, one can extract a field spanning
a doubleword and then insert it into another pair of doublewords.
1 pseudo-instruction, 2 actual instructions.

Idea here is that it may do, essentially a combination of a shift and a
masked bit-select, say:
   Low 8 bits of immediate encode a shift in the usual format:
     Signed 8-bit shift amount, negative is right shift.
   High bits give a pair of bit-offsets used to compose a bit-mask.
     These will MUX between the shifted value and another input value.

You want the offset (a 6-bit number) and the size (another 6-bit number)
in order to identify the field in question.

I am still not sure whether this would make sense in hardware, but is
not entirely implausible to implement in the Verilog.

In the extract case, you have the shifter before the masker
In the insert case, you have the masker before the shifter
followed by a merge (OR). Both maskers use the size. Offset
goes only to the shifter.

Would likely be a 2 or 3 cycle operation, say:
   EX1: Do a Shift and Mask Generation;
     May reuse the normal SHAD unit for the shift;
     Mask-Gen will be specialized logic;
   EX2:
     Do the MUX.
   EX3:
     Present MUX result as output (passed over from EX2).

I have done these in 1 cycle ...