CPU REGISTERS
This section will cover the programmable CPU registers. The Assembly
language programmer must be familiar with the references and uses of each
register in order to program effectively. Other registers are discussed in the
section on different 80X86 processors.
The following is a list of the 16-bit registers and their references:
|
AX
|
Accumulator
|
|
FL
|
Flags
|
|
|
X|X|X|X|OF|DF|IF|TF|SF|ZF|X|AF|X|PF|X|CF
|
|
BX
|
Base Index
|
|
BP
|
Base Pointer
|
|
CX
|
Counter
|
|
DX
|
Data I/O Index
|
|
DI
|
Destination Index
|
|
SI
|
Source Index
|
|
SP
|
Stack Pointer
|
|
IP
|
Instruction Pointer
|
|
CS
|
Code Segment
|
|
SS
|
Stack Segment
|
|
DS
|
Data Segment
|
|
ES
|
Extra Segment
|
Some registers can be divided and accessed as two eight-bit registers for
byte operations. They are referenced as follows:
|
AH
|
AX
|
High byte
|
|
AL
|
AX
|
Low byte, eight bit accumulator
|
|
BH
|
BX
|
High byte
|
|
BL
|
BX
|
Low byte
|
|
CH
|
CX
|
High byte
|
|
CL
|
CX
|
Low byte, shift counter
|
|
DH
|
DX
|
High byte
|
|
DL
|
DX
|
Low byte
|
AX and AL are the Accumulator
AX = AH+AL. AX is the 16-bit
reference for the accumulator; AL is the 8-bit reference. AL is the low half of
AX with AH as the high half. The accumulator is the primary data register. Most
instructions for handling data execute faster if the data is in the
accumulator.
FL Flag Register
The Flag register is a 16-bit data
register used to keep track of CPU activity. This includes all logical,
arithmetic, and comparing results as well as interrupt controls, debug tracing,
string direction flags, etc.
Most conditional jumping instructions use the contents of this register to
determine if branch conditions are true or not true.
Flag Bits X|X|X|X|OF|DF|IF|TF|SF|ZF|X|AF|X|PF|X|CF
The bit positions with X are not
defined for the 8086/8088 CPU but are reserved by Intel for use with other
processors in the series.
OF Overflow Flag
This bit is set if the last data
manipulation caused the high bit to change.
DF Direction Flag
This bit is used by the CPU to
decide the direction of string operations. Clearing the bit causes string
operations to go forward and setting the bit causes string operations to go
backward.
IF Interrupt Flag
This bit can be set or cleared by
the programmer to prevent or allow maskable interrupts to occur.
TF Trace Flag
This bit is used in the debugging
mode for single stepping through program logic.
SF Sign Flag
This bit is reset by logical
operations to be equal to the high bit of the resulting data.
ZF Zero Flag
This bit is set if the last data
manipulation produced a zero condition.
AF Aux Carry Flag
This bit is used by logical
instructions that deal with data in nibbles (four bit).
PF Parity Flag
This bit is reset by the last data
manipulation instruction to reflect if the operation produced an even or odd
parity condition. A 1 means even parity and a 0 means odd parity.
CF Carry Flag
When adding, the carry bit is set
if an overflow occurs. If subtracting, the bit is set if it had to borrow a bit
because the subtraction resulted in a sign flip.
BX Base Index
BX is the most flexible of the
indexing registers. It is a 16-bit register that can also be addressed in 8-bit
format as BH (high) and BL (low) where BX = BH+BL. BX may beadded to other
index registers for working with morecomplex indexing offsets. Examples:
[BX+offset], [BX+SI+offset], [BX+DI+offset]
BP Base Pointer
BP is the base pointer register
used to index data in the stack area. This register is used by many compilers
to index data frames in the stack area. BP may be combined with DI or SI to index
data in the stack area. Examples: [BP+SI+offset], [BP+DI+offset]
SI Source Index
SI is the source index register
used by the string instructions. It may be combined with BX or BP to index
data. Example: [SI+BX+offset]
DI Destination Index
DI is the destination index
register used by the string instructions. It may be combined with BX or BP to
index data. Example: [DI+BX+offset]
SP Stack Pointer
SP is the stack pointing register
used by the push, pop, call, interrupt, and return instructions. It always
indexes the last word pushed onto the stack.
CX Counter
CX is the counter register used by
the string, repeat, and loop instructions.
DX Data Register, I/O index
DX is the data register. DX is only
used as an index for I/O port functions. It is used for the 16 by 16 bit
multiply and the 32 by 16 bit divide instructions. The results from a 16 bit
multiply are put into DX:AX where DX holds the high 16 bits and AX holds the
low 16 bits. For a 32 bit by 16 bit divide, DX will hold the leftover (modulo)
data resulting from the divide.
IP Instruction Pointer
IP is used to index the next
instruction to execute. It is reset by call and jump instructions.
