Look at the format of the 68000's MOVEQ instruction. The zero is part of the instruction, and does not take an extra four bytes to hold it. There's no memory that's used (other than the instruction itself), no extra memory to hold the argument, and no "always zero" register.
MOVEQ can move more than a zero. It can move any small number (-128 to 127), so 0 is not "special" here.
Also check out the CLR instruction (though that may be what you meant by "a mnemonic for some or all of the instructions that assume zero").
MOVEQ can move more than a zero. It can move any small number (-128 to 127), so 0 is not "special" here.
Also check out the CLR instruction (though that may be what you meant by "a mnemonic for some or all of the instructions that assume zero").