28 Apr
2024
28 Apr
'24
10:59 p.m.
After learning more about the PALM processor in the IBM 5100, it has a
similarity to the 6502 in that the first 128 bytes of RAM is a "register
file." All its registers (R0 to R15, across 4 interrupt "layers") occupy
those first addresses. In addition, they are physically on the processor
itself (not in actual RAM). I've been meaning to come up with a sample
PALM program that verifies if there is any performance advantage on that
(that is, something that "does stuff" with data in addresses 0 to 127, then
"does that same stuff" in a higher address like $800+ and see if there is a
noticeable performance difference). The earliest document I can find on
PALM is from 1972 (or just a few months after the 8080 - the actual initial
production date of PALM is unknown, the 1972 date is just when IBM
documented the instruction set). But I think the IBM System/3 had a
similar design (or at least, just I recall a mention of the System/3
registers are in RAM -- not sure if that's literal or just
address-access-wise, but in any case the System/3 was said to be pretty
difficult to program for).
Anyhow, to me the PALM may be an earlier "RISC" approach in that its
instructions are always 2-bytes (4-bits for a main opcode -- yes only 16
categories, then a few bits for a "modifier" while the middle pair of bits
specify a register R0 to R15 that the instruction involves), in contrast to
the variable instruction length used by the System/360. There is one
exception in PALM where a kind of "long jump" instruction is followed by
another 2-bytes that is the target address.
You won't hear much about PALM - though I am excited that emulation support
for it has recently been added into MAME! I ponder if maybe Chuck Peddle
somehow crossed paths with PALM in his early engineering career, or if in
some indirect way there was some lineage or connection there (in a "dude,
your processor doesn't need to be that complicated, I know a system that in
16 instructions does all sort of stuff" kind of way). BTW, IBM's costs
list during the IBM SCAMP development put the PALM processor card costing
about $300 (in c. 1973).
All that said, in early 1970s, I don't think anyone was yet using the term
RISC vs CISC.
-Steve v*
On Sun, Apr 21, 2024 at 7:50 PM Peter Coghlan via cctalk <
cctalk(a)classiccmp.org> wrote:
My first exposure to a computer at home was a BBC
Micro with 32kB of RAM
and
32kB of ROM. Included in this was a 16kB BASIC ROM which was regarded as
fast
and powerful, featuring 32 bit integer variables, 40 bit floating point
variables, variable length strings, structured programming constructs and
everything accessed by keyword statements rather than PEEK this and POKE
that.
This was implemented by a humble 6502 running at (mostly) 2MHz, with one 8
bit
arithmetic register, two 8 bit index registers, one 8 bit stack pointer,
a 16 bit program counter and a few flag bits.
I would have expected that a computers featuring a Z80 with its larger
register
set, 16 bit arithmetic operations, richer instruction set and general bells
and whistles would have been able to produce a much superior
implementation in
terms of speed or features or both but I never came across one.
Why is that? Did the Z80 take more cycles to implement it's more complex
instructions? Is this an early example of RISC vs CISC?
Regards,
Peter Coghlan