Some unlikely 2021 predictions

Tell that to CPU architects. Motorola's 6800 had the same three shift instructions as the Z-80. The 80x86 processors also only have three shift instructions. The 8086 User's Manual states: "SHL and SAL (Shift Logical Left and Shift Arithmetic Left) perform the same operation and are physically the same instruction. ARM has the LSR, LSL, and ASR. RISC-V has SRLI, SLLI, and SRAI (shift right/left logical/arithmetic immediate). MIPS? Same thing.

It is a well known fact that an arithmetic shift only differs from a logical shift when shifting "right", i.e. towards the less significant bit positions. This has been known for decades, and I doubt the engineers at Zilog were stupid when they added these instructions. So to me, the *obvious* explanation was that Zilog copied the three instructions from the Motorola 6800, and left the op code that was left unused to do what it happened to do.

What Zilog *should* have done was to do what Intel did later with the 8086: document the assembler mnemonic SLL as an alias for SLA. Or used the name SLL instead of SLA.

>And ime, not being logically complete is *asking for trouble*. What happens if an assembler-writer uses SRL, and then also uses SLL because it's the logically obvious instruction?

What *I* think is asking for trouble is when an assembler writer jumps to conclusions about the instruction set. "Ah, there is an error in the documentation, Zilog forgot to document the SLL instruction. Ok, let's use the *obvious* opcode for that." And the ship the assembler without testing it.

>Do you have any *evidence* as to why SLL wasn't documented?

Do *you* have any evidence for your original claim? That Zilog didn't intend to design their processor according to industry practice at the time, and they really intended to define four shift operations, of which two are identical? And with a stroke of extremely good luck they could just drop the SLL instruction from the documentation, when they realized at the last moment that it didn't work as expected. What are the odds of that?