Question

In Intel architecture IA32, instructions like movl, movw does not allow operands that are both memory locations. For example, instruction movl (%eax), (%edx) is not permitted. Why?

Answer 1

The answer involves a fuller understanding of RAM. Simply stated, RAM can only be in two states, read mode or write mode. If you wish to copy one byte in ram to another location, you must have a temporary storage area outside of RAM as you switch from read to write.

It is certainly possible for the architecture to have such a RAM to RAM instruction, but it would be a high level instruction that in microcode would translate to copying of data from RAM to a register then back to RAM. Alternatively, it could be possible to extend the RAM controller to have such a temporary register just for this copying of data, but it wouldnt provide much of a benefit for the added complexity of CPU/Hardware interaction.

EDIT: It is worth noting that recent advancements such as Hybrid Memory Cube and High Bandwidth Memory are achitectures in which the RAM topology has become more like PCI-e and direct RAM to RAM transfers are now possible, but that is due to the support logic for the technologies, not the RAM itself. In the CPU architecture, this would be in the form of huge blocks of RAM at a time, like DMA, and not in the form of a single instruction, plus the CPU cache behaves like traditional RAM so the architecture would have to abstract it as per my original explanation

EDIT2: Per @PeterCordes comment, my original understanding was not entirely correct; x86 does in fact have a few memory to memory instructions. The real reason they are not available for most instructions (such as movl and movw) is to keep instruction encoding complexity low, but they could have implemented them. However, the basic idea in my original answer, that there is a temporary storage location outside of RAM in the form of a latch or register, is correct, but the idea that this is the reason why these instructions don’t exist is not. Even older chips from the 1970s such as the 6502 and the 8086 have memory to memory instructions, and you could easily perform operations such as INC directly on a RAM location. This was accomplished by latching the memory fetch directly to the ALU and back out to memory again without going through a register used by the instruction set.