of Microprocessors

MICROPROCESSORS 169
Instruction Set
The 68000 instruction set is best described as simple and straightforward.
Although there are only 56 distinct instruction types (just a few more
than the 6502), there are thousands of variations when one takes into account
the addressing modes and other instruction "modifiers." The instruction set
is claimed to be highly "orthogonal" or "symmetrical," which in the limit
means that any instruction could use any addressing mode for both source
and destination operands. Since actual achievement of this ideal would
require a very large number of bits in the operation code and lead to many
nonsense instructions, most 68000 insrructions have some kind ofaddressing
mode restrictions. Occasionally, these are severe, for instance, the program
counter relative mode can never be used to address a destination.
Besides one or two addressing mode specifications, most instructions
have a size modifier that specifies the length of the operands (Table 5-4). The
available sizes are byte (8 bits), word (16 bits), and long word (32 bits). Some
instructions may have an implied size or may operate only on words or long
words. When either a data or an address register is used as a source operand,
the size modifier specifies how much of the lower part of the register to read.
When a data register is used as a destination, only the lower part is changed.
All 32 bits of an address register, however, receive the sign-extended result of
any instruction using it as a destination.
One reason for so few instruction types for such a powerful processor is
the presence of a generalized "Move" instruction. This instruction can use
any addressing mode or register for the source and almost any addressing
mode for the destination. So important is Move that nearly one-quarter of the
possible combinations of the 16 operation code bits are used by it. With
appropriate addressing modes, Move replaces the myriad loads, stores,
pushes, pops, etc., of other instruction sets. One of the variations of Move
uses what is known as a "quick immediate" source operand. In a single 16-bit
instruction word, any value between - 128 and + 127 can be loaded into any
data register. A "move multiple registers" instruction can move any combination
of the general registers to or from any memory location using any of the
memory-addressing modes. The combination of registers desired is specified
using a one-word bit map where each bit corresponds to a register. This
instruction really makes saving and restoring registers in subroutines easy,
and it executes at full bus-limited speed.
Add and subtract binary arithmetic can be performed on 8-, 16-, and
32-bit quantities. Not only can a register be the recipient of the result, but a
register can be added to memory with the result stored in memory. There is
no provision in the 68000 for pairing up data registers to hold 64-bit
operands or results so multiply is restricted to 16 X 16 and divide to 32/16.
Both signed and unsigned binary arithmetic is possible as well as binarycoded-decimal
add and subtract. "Add quick" and "subtract quick" allow any
register or memory location to be incremented or decremented by 1 to 8 with