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String instructions automatically update the SI register, the DI register, or both, before processing
the next string element. The Direction Flag (DF) determines whether the index registers are auto-
incremented (DF = 0) or auto-decremented (DF = 1). The processor adjusts the DI, SI, or both
registers by one for byte strings or by two for word strings.
If a repeat prefix is used, the count register (CX) is decremented by one after each repetition of
the string instruction. The CX register must be initialized to the number of repetitions before the
string instruction is executed. If the CX register is 0, the string instruction is not executed and
control goes to the following instruction.
Table 2-8. String Instruction Register and Flag Use
2.2.1.5
Program Transfer Instructions
The contents of the Code Segment (CS) and Instruction Pointer (IP) registers determine the in-
struction execution sequence in the 80C186 Modular Core family. The CS register contains the
base address of the current code segment. The Instruction Pointer register points to the memory
location of the next instruction to be fetched. In most operating conditions, the next instruction
will already have been fetched and will be waiting in the CPU instruction queue. Program transfer
instructions operate on the IP and CS registers. Changing the contents of these registers causes
normal sequential operation to be altered. When a program transfer occurs, the queue no longer
contains the correct instruction. The Bus Interface Unit obtains the next instruction from memory
using the new IP and CS values. It then passes the instruction directly to the Execution Unit and
begins refilling the queue from the new location.
The 80C186 Modular Core family offers four groups of program transfer instructions (see Table
2-9). These are unconditional transfers, conditional transfers, iteration control instructions and in-
terrupt-related instructions.
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
SI
Index (offset) for source string
DI
Index (offset) for destination string
CX
Repetition counter
AL/AX
Scan value
Destination for LODS
Source for STOS
DF
Direction Flag
0 = auto-increment SI, DI
1 = auto-decrement SI, DI
ZF
Scan/compare terminator
2-23
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