Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 206)

192 Assembly Language Programming for the 68000 Family

The CMPI and TST instructions now allow all of the PC relative

addressing modes. However, this extension only applies when word or

longword operands are used.

The divide and multiply instructions, DIV and MUL, have been

greatly expanded. Recall that the four forms of these instructions were:

DIVU <ea>,Dn

DIVS <ea>,Dn

MULU <ea>,Dn

MULS <ea>,Dn

The divide instructions require a 16-bit source and 32-bit destination

operand. The result is a 32-bit value consisting of a 16-bit quotient

and a 16-bit remainder. The multiply instructions multiply two 16-bit

operands, yielding a 32-bit result. The expanded divide instructions have

the following forms:

DIVS.W <ea>,Dn DIVU.W <ea>,Dn 32/16->16r:16q

DIVS.L <ea>,Dq DIVU.L <ea>,Dq 32/32->32q

DIVS.L <ea>,Dr:Dq DIVU.L <ea>,Dr:Dq 64/32->32r:32q

DIVSL.L <ea>,Dr:Dq DIVUL.L <ea>,Dr:Dq 32/32->32r:32q

The operation of these instructions is relatively straightforward. The

first form is the form found on the 68000. This is the default if no

size is specified. The three extended forms vary in the sizes of the

operands and/or the result. Notice that a register pair is required for

operands/results that require more than 32 bits total. The notation Dr/Dq

refers to the quotient and remainder registers. These can be any of the

data registers.

The expanded multiply instructions have the following format:

MULS.W <ea>,Dn MULS.W <ea>,Dn 16Xl6->32

MULS.L <ea>,Dl MULS.L <ea>,Dl 32X32->32

MULS.L <ea>fDh:Dl MULS.L <ea>,Dh:Dl 32X32->64

The first form is the old 68000 form. In this case two 16-bit operands

are multiplied to yield a 32-bit result. The expanded forms allow the

multiplication of two 32-bit operands to yield either a 32-bit or a 64-bit

result. In the case of a 64-bit result, Dh and Dl refer to the registers used

to hold the high-and low-order 32 bits of the result.

The EXT instruction has been expanded. Recall that the EXT.W sign

extends a byte to a word, and the EXT.L sign extends a word to a

longword. In order to sign-extend a byte to a longword, both of these

instructions had to be used. The 68020 allows a sign extension from byte

to longword with a single instruction. A slightly different mnemonic has to

be introduced. This is not really a different instruction, but an expanded

form of EXT. The new instruction format is EXTB.L. This is the exact

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Motorola M68000 Betriebsanweisung Seite 206