Electronic – early accumulator based machines

computer-architecturecpuregister

Wikipedia says that many of the early machines were accumulator-based machines.
My guess is that in those machines the accumulator did not play the role of a speed-up register as do registers in today load and store architectures.

The accumulator had mainly the role of implicit operand so that instructions could be shorter
(one operand of an arithmetic operation always resided there and also its result was stored in the accumulator)

Is my guess right ?

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I think it depends on the machine, its design aims and the limitations of the technology.

Most traditional accumulator machines have very few registers, and many only have a single ‘general-purpose’ register (by today's standards). On the PDP-8, for example, there are a few registers, but the only one you can access directly is the AC. The vast majority of instructions (not that ‘vast’ applies to a design with eight instructions) operate on the AC. The only other object to operate on is memory, so most instructions have a single field (a 7-bit address). The data transfer direction is implicit in the instruction (think load/store).

On the other hand, the machine languages of many accumulator machines were a side effect of the system architecture more so than the other way round (which is how we do it today, software being king). The PDP-8 was an accumulator machine because, well, 12 bits worth of flip-flops were pretty expensive equipment in the late 60s and the PDP-8 was a cheap computer.

As for the speed factor: in-processor registers were almost always faster than memory, same as now. But their cost was much higher than it is now, instruction widths were smaller, sequencers and state machines were much simpler, and so there weren't many registers. Unless you have at least two of them, you can keep your instruction set very simple. And vice versa.

Also, with respect to early computers using ‘accumulators’ instead of ‘registers’: ‘accumulator’ came from its use in tabulating machines to denote registers for running totals. Presumably operators would have been more comfortable with the term. The PDP-8 schematics refer to every single flip-flop in the system as a ‘register’, but there's only one ‘accumulator’ and it's marked as a ‘major’ register.

Related Solutions

Electronic – Implementing multiplication

At minimum, you need a branching instruction which can determine whether the previous addition generated a carry. Also, performance will be improved enormously if you expand your register set to three or four registers.

Start with one operand in R0 and the other in R1. Start with R2 zero.

Repeat the following sequence eight times (first ADD may be skipped on first pass)

  Add R2,R2
  Add R0,R0
  Bnc Skip
  Add R2,R1
Skip:

Note that if the instruction set includes an add-with-carry instruction, using it for the second instruction would cause this instruction sequence to yield a 16-bit result in R0:R2. Further, if R2 starts non-zero, its value times 256 will be added to the outgoing result.

Electrical – Load/Store architecture

What you mention is only one of many differences between CISC and RISC.

One way that RISC tries to minimize memory stalls is by having the compiler schedule the memory accesses. With CISC, the compiler has little opportunity to optimize memory accesses, but an advantage of RISC's simpler, single-cycle instructions is that it can rearrange those instructions at compile-time to optimize memory accesses. CISC instructions are too complex for the compiler to know when and where instructions can be rearranged. RISC's advantage partially depends on an optimizing compiler understanding how instruction flow can be manipulated.

There are other attributes of RISC that are meant to offer improvements. One is heavy pipe-lining, another is the potential for faster clock speeds, and a third is instruction and data caches. However, CISC architectures have adopted many of the techniques that were envisioned for RISC, and have tended to keep up with RISC in performance.

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Related Solutions

Electronic – Implementing multiplication

Electrical – Load/Store architecture

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