Re: Cost of handling misaligned access

On 2/4/2025 1:25 PM, Scott Lurndal wrote:

mitchalsup@aol.com (MitchAlsup1) writes:

On Tue, 4 Feb 2025 4:49:57 +0000, EricP wrote:
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MitchAlsup1 wrote:

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Basically, VAX taught us why we did not want to do "all that" in
a single instruction; while Intel 432 taught us why we did not bit
aligned decoders (and a lot of other things).

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I case people are interested...
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[paywalled]
The Instruction Decoding Unit for the VLSI 432 General Data Processor,
1981
https://ieeexplore.ieee.org/abstract/document/1051633/
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The benchmarks in table 1(a) below tell it all:
a 4 MHz 432 is 1/15 to 1/20 the speed (slower) than a 5 MHz VAX/780,
1/4 to 1/7 speed than a 8 MHz 68000 or 5 MHz 8086
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A Performance Evaluation of The Intel iAPX 432, 1982
https://dl.acm.org/doi/pdf/10.1145/641542.641545
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And the reasons are covered here:
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Performance Effects of Architectural Complexity in the Intel 432, 1988
https://www.princeton.edu/~rblee/ELE572Papers/Fall04Readings/I432.pdf

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From the link::
The 432’s procedure calls are quite costly. A typical procedure call
requires 16 read accesses to memory and 24 write accesses, and it
consumes 982 machine cycles. In terms of machine cycles, this makes
it about ten times as slow as a call on the MC68010 or VAX 11/780.
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almost 1000 cycles just to call a subroutine !!!
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Lots of thinigs teh architects got wrong in there.....

 While true, it's easy to say in retrospect after forty+
years of advancements in silicon design and technology.
 Comparing to the CISC architectures of the 60s and 70s,
it's not horrible.

Well, vs a modern RISC style ISA, say, caller side:
   MOV  R20, R10  //0c (SSC with following)
   MOV  R21, R11  //1c
   BSR  func     //2c (typically)
Cost: 3 cycles.
func:
   ADD SP, -32, SP      //2c (1 c penalty)
   MOV.Q  LR, (SP, 24)  //1c
   MOV.X  R18, (SP, 0)  //1c
   ...
   MOV.Q  (SP, 24), LR  //2c (1c penalty)
   MOV.X  (SP, 0), R18  //1c
   JMP    LR            //10c (*1)
*1: Insufficient delay since LR reload, so branch predictor fails to handle this case.
Cost: 16 cycles.
...