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TI486 Microprocessor - Al Kossow's Bitsavers

TI486 Microprocessor - Al Kossow's Bitsavers

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Instruction Fields<br />

Table 7-2. Instruction Prefix Summary<br />

7.2.2 Opcode Field<br />

Table 7-2 lists the encodings for each of the available prefix bytes. The<br />

operand size and address size prefixes allow the individual overriding of the<br />

default value for operand size and effective address size. The presence of<br />

these prefixes select the opposite (non-default) operand size and/or effective<br />

address size as the case may be.<br />

PREFIX ENCODING DESCRIPTION<br />

ES: 26h Override segment default, use ES for memory operand.<br />

CS: 2Eh Override segment default, use CS for memory operand.<br />

SS: 36h Override segment default, use SS for memory operand.<br />

DS: 3Eh Override segment default, use DS for memory operand.<br />

FS: 64h Override segment default, use FS for memory operand.<br />

GS: 65h Override segment default, use GS for memory operand.<br />

Operand size 66h Make operand size attribute the inverse of the default.<br />

Address size 67h Make address size attribute the inverse of the default.<br />

LOCK FOh Assert LOCK hardware signal.<br />

REPNE F2h Repeat the following string instruction.<br />

REP/REPE F3h Repeat the following string instruction.<br />

The opcode field is either one or two bytes in length and specifies the operation<br />

to be performed by the instruction. Some operations have more than one<br />

opcode, each specifying a different form of the operation. Some opcodes<br />

name instruction groups. For example, opcode Ox80 names a group of<br />

operations that have an immediate operand, and a register or memory<br />

operand. The group opcodes use an opcode extension field of 3 bits in the<br />

following byte, called the MOD RIM byte, to resolve the operation type.<br />

Opcodes for the entire <strong>TI486</strong>SLC/DLC instruction set are listed in the<br />

Instruction Set Summary Table. The opcodes are given in hex values unless<br />

shown within brackets ([ ]). Values shown in brackets are binary values.<br />

7.2.3 vv Field<br />

The 1-bit field indicates the operand size during 16- and 32-bit data operations.<br />

Table 7-3. w Field Encoding<br />

wFIELD<br />

0<br />

1<br />

OPERAND SIZE<br />

OPERAND SIZE<br />

16-BIT DATA OPERATIONS 32-BIT DATA OPERATIONS<br />

8 bits 8 bits<br />

16 bits 32 bits<br />

7.2.4 d Field<br />

The d field determines which operand is taken as the source operand and<br />

which operand is taken as the destination.<br />

Table 7-4. d Field Encoding<br />

d FIELD DIRECTION OF OPERATION SOURCE OPERAND DESIGNATION OPERAND<br />

0 Register ---7 Register/Memory reg mod rim or mod ss-index-base<br />

1 Register/Memory ---7 Register mod rim or mod ss-index-base reg<br />

7-6 Instruction Set

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