AMD Am186 Series, Instruction Set, Page: 35 / 320

Instruction Set Listing 3-9
3.1.11 Processor Control
Processor Control Instructions
Coprocessor Interface Instructions
3.1.12 String
A string is a contiguous sequence of components stored in memory. For example, a string
might be composed of a list of ASCII characters or a table of numbers.
A string instruction operates on a single component in a string. To manipulate more than
one component in a string, the string instruction
prefixes (REP/REPE/REPNE/REPNZ/
REPZ) can be used to repeatedly execute the same string instruction.
A string instruction uses an index register as the offset of a component in a string. Most
string instructions operate on only one string, in which case they use either the Source
Index (SI) register or the Destination Index (DI) register. String instructions that operate on
two strings use SI as the offset of a component in one string and DI as the offset of the
corresponding component in the other string.
After executing a string instruction, the microcontroller automatically increments or
decrements SI and DI so that they contain the offsets of the next components in their strings.
The microcontroller determines the amount by which the index registers must be
incremented or decremented based on the size of the components.
The microcontroller can process the components of a string in a forward direction (from
lower addresses to higher addresses), or in a backward direction (from higher addresses
to lower ones). The microcontroller uses the value of the Direction Flag (DF) to determine
whether to increment or decrement SI and DI. If DF is cleared to 0, the microcontroller
increments the index registers; otherwise, it decrements them.
String-Instruction Prefixes
Mnemonic Name See Page
HLT Halt 4-57
LOCK Lock the Bus 4-140
NOP No Operation 4-165
Mnemonic Name See Page
ESC Escape 4-56
WAIT Wait for Coprocessor 4-245
Mnemonic Name See Page
REP Repeat 4-191
REPE Repeat While Equal 4-193
REPNE Repeat While Not Equal 4-197
REPNZ Repeat While Not Zero ( Synonym for REPNE) 4-197
REPZ Repeat While Zero ( Synonym for REPE) 4-193