🚀 go-pugleaf

With all the postings about bitfield ops, I thought I'd better give a
quick and dirty overview of HP-PA's operations. The rationale for having
bitfield ops at all was partly bitmap data, partly Pascal packed structures,
partly doing FP in software, & a lot of OS packed data structures.....

Extract [Signed] [Variable] - 
   Right justify a field, given a bit Position of the field's LSB as an
   immediate, and the field length as an immediate.
   The [Signed] option sign-extends instead of zero extends.
   The [Variable] option takes the LSB bit position from a special register.

This instruction can be used for right shifts of various flavors. If
(LSB pos_leng>32, then MSB of reg is used as MSB of field.

The rationale for having [variable] take a position out of a register is:
- expected critical path in the ALU (possibly not the case in CMOS)
- when a variable position is used, it's often used over & over, so having
  to move the position to a special register is not a significant overhead.

Note that there is no variable length. We didn't see it happening enough to
put it in, and in the rare cases it occurs, a case branch can be used.

[Zero & ][Variable] Deposit [Immed.] -
   Take a Right justified field and put it back into its (unjustified)
   position, given a bit position of the LSB as an immediate,
   and a Field Length as an immediate.

   The [Zero] option clears the destination register, otherwise bits that
   are not part of the field are left unchanged. 
   The [Variable] option takes the LSB bit position from a special register.
   The [Immed] option uses a five bit signed immediate as a source.

With the [Zero] option, this can be used for left shifts.
With the Immediate option, this can be used as a set or clear bit (or field),
 (fairly common in OS structures).
Using both options, constants can be constructed.
Since something like 80% of immediates are in the range -16:+16,
 this restricted immediate is still quite useful. 

[Variable] Shift Double
  Concatenate two registers, right shift the 64 result by an amount specified
  by an immediate, and store the 32 low order bits into the result.
 
   The [Variable] option takes shift amount from a special register.

This was intended to be used to justify/position fields that crossed word
boundaries.


While this sounds like a lot of instructions and variations, the hardware
required to implement it is quite simple. Maybe someone from the HP-PA
team can comment on whether it is worthwhile (i.e. used much).