AMD Am186 Series, Instruction Set

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Instruction Set 4-238
STOS STOS
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.
if (size( destination) == 8)
/* store bytes */
{
ES:[DI] = AL;
if (DF == 0) /* forward */
increment = 1;
else /* backward */
increment = -1;
}
if (size( destination) == 16)
/* store words */
{
ES:[DI] = AX;
if (DF == 0) /* forward */
increment = 2;
else /* backward */
increment = -2;
}
/* point to location for next string component */
DI = DI + increment;
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 AL,ASTERISK ; copy character to AL
LEA DI,STRING ; load offset (segment = DS)
MOV CX,LENGTH STRING ; set up counter
CLD ; process string low to high
; fill string
REP STOSB