Instruction Set
4-231
STD
Set Direction Flag
STD
What It Does
STD sets the Direction Flag (DF) to 1, causing subsequent
string instructions to process
the components of a string from a higher address to a lower address.
Syntax
Description
STD sets the Direction Flag, causing all subsequent string operations to decrement the
index registers on which they operate, SI or DI or both.
Operation It Performs
Flag Settings After Instruction
Form
Opcode
Description
Clocks
Am186 Am188
STD
FD
Set the Direction Flag so the Source Index (SI) and/or
the Destination Index (DI) registers will decrement during
string instructions
2
2
STD
/* process string components from higher to lower addresses */
DF = 1;
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
–
1
–
–
–
– res – res – res –
Summary of Contents for Am186 Series
Page 1: ...Am186 and Am188 Family Instruction Set Manual February 1997...
Page 10: ...Table of Contents x...
Page 18: ...Programming 1 8...
Page 40: ...Instruction Set Listing 3 14...
