AMD Am186 Series, Instruction Set

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Instruction Set 4-133
LEA Load Effective Address LEA
What It Does
LEA loads the offset of a memory component into a register.
Syntax
Description
LEA calculates the effective address (offset part) of the component and stores it in the
specified register.
Operation It Performs
Flag Settings After Instruction
Examples
This example fills a string in memory with a character. Because the Direction Flag (DF) is
cleared to 0 using CLD, the bytes are filled, one by one, from first to last.
Form Opcode Description
Clocks
Am186 Am188
LEA
r16,m16
8D
/r
Load offset for m16 word in 16-bit register 6 6
LEA offset,component
/* copy offset of component */
offset = &component;
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Processor Status
Flags Register
reserved
OF DF IF TF SF ZF AF PF CF
? = undefined; – = unchanged
– – – – – – res – res – res –
STRING DB 128 DUP (?)
ASTERISK DB ’*’ ; 2Ah
; fill string with character
; set up registers and flags
MOV AX,SEG STRING
MOV ES,AX
MOV AL,ASTERISK ; copy character to AL
LEA DI,STRING ; load offset (segment = ES)
MOV CX,LENGTH STRING ; set up counter
CLD ; process string low to high
; fill string
REP STOSB