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This is the R.O.M.E.68000 Manual version 0.51.
This is an experimental version.
Copyright © 1997 by G. Mezzetti; last update: January 1, 1999.

5. How Multiple Operands Are Described

We have just said that, for each instruction, the description of that instruction tells you the kind of each of its operands. This is done by means of some symbols that we are now going to describe. These symbols are the dash (``Ð''), the letter ``v'', or else a number. They are listed below along with their meaning.

Symbol: ``Ð''

The operand is a simple operand, and is fetched from memory as usual.

Symbol: ``v''

The operand is a multiple operand, and is contained in a variable-size block. The operand is fetched from memory and, if necessary, rewritten back to it one component at a time, during the body of the execution of the instruction. The length of the operand (i.e., of the block of words containing it) is given by operand L of the same instruction, but cannot be greater than the current limit for the size of variable-size blocks (if it is greater, an error is generated and the program executing the instruction is killed).

Symbol: a number n

The operand is a multiple operand, and is contained in a fixed-size block. The operand is fetched from memory and, if necessary, rewritten back to it one component at a time, during the body of the execution of the instruction. The length of the operand (i.e., of the block of words containing it) is implicit in the instruction, and is given by the number n; the number n can be greater than the current limit for the size of variable-size blocks, since this limit does not affect fixed-size ones.

Important remark: although the limit for the size of variable-size blocks must be a power of two, the length of such a block is by no means bounded to be also a power of two: it can be any number from zero (inclusive) to the current limit (inclusive too). As for the uses of operands, wait until you see these symbols at work.

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