[logicalshift]

The main difference between the ARM1 (which was never sold in any quantity) and the ARM2 was the addition of the multiply instruction (and the multiply and accumulate instruction). They were the only multi-cycle arithmetic operation they had, and what's more you couldn't load arbitrary constants into a register with the MOV instruction or use constants with the multiply instruction itself so they were still a bit inconvenient as well as comparatively slow.

But you could write:

  MOV R1, #3
  MUL R0, R0, R1

In spite of the limitations, this gave the instruction set a certain 68000 quality to it (except much faster for a given clock speed).

To multiply the number in R0 by 3, which was pretty convenient. The ARM had a thing called the barrel shifter though, which let you add an arbitrary shift to the last operand of any arithmetic operation. All arithmetic ops take 1 processor cycle, so you could write this instead to multiply by 3 in a single cycle:

  ADD R0, R0, R0, LSL #1

Ie, add R0 to itself multiplied by 2. Constant divisions could be constructed with the SUB instruction too. Some constants required multiple instructions (but I think the maximum was something like 4 or 5 instructions for any constant? I wrote an assembler that could figure this out for you automatically in the mid-90s so I used to know for sure).

This is basically a single-instruction version of the 6502 trick (handy, because the first OS for the ARM was a hurried port of a 6502 operating system), which sort of fits with the ARM's original inspiration as being a 32-bit version of the 6502. As each instruction completes in one CPU cycle, the ARM could have fairly monstrous integer performance for the mid to late 80s if you knew how to program it.