Motorola M68000 User Manual download pdf (Page 9)

Addressing Capabilities

2-2

M68000 FAMILY PROGRAMMER’S REFERENCE MANUAL

MOTOROLA

An instruction speciﬁes the function to be performed with an operation code and deﬁnes the

location of every operand. Instructions specify an operand location by register speciﬁcation,

the instruction’s register ﬁeld holds the register’s number; by effective address, the

instruction’s effective address ﬁeld contains addressing mode information; or by implicit

reference, the deﬁnition of the instruction implies the use of speciﬁc registers.

The single effective address operation word format is the basic instruction word (see Figure

2-2). The encoding of the mode ﬁeld selects the addressing mode. The register ﬁeld

contains the general register number or a value that selects the addressing mode when the

mode ﬁeld contains opcode 111. Some indexed or indirect addressing modes use a

combination of the simple effective address operation word followed by a brief extension

word. Other indexed or indirect addressing modes consist of the simple effective address

operation word and a full extension word. The longest instruction is a MOVE instruction with

a full extension word for both the source and destination effective addresses and eight other

extension words. It also contains 32-bit base displacements and 32-bit outer displacements

for both source and destination addresses. Figure 2-2 illustrates the three formats used in

an instruction word; Table 2-1 lists the ﬁeld deﬁnitions for these three formats.

SINGLE EFFECTIVE ADDRESS OPERATION WORD FORMAT

BRIEF EXTENSION WORD FORMAT

FULL EXTENSION WORD FORMAT

Figure 2-2. Instruction Word Specification Formats

1514131211109876543210

XXXXXXXXXX

EFFECTIVE ADDRESS

MODE REGISTER

1514131211109876543210

D/A REGISTER W/L SCALE 0 DISPLACEMENT

1514131211109876543210

D/A REGISTER W/L SCALE 1 BS IS BD SIZE 0 I/IS

BASE DISPLACEMENT (0, 1, OR 2 WORDS)

OUTER DISPLACEMENT (0, 1, OR 2 WORDS)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 ... 91 92

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Motorola M68000 User Manual Page 9