Texas Instruments TMS320C28x, Reference Manual

Page: 236 / 695

CMPB AX, #8bit
6-79
CMPB AX, #8bit Compare 8-bit Value
SYNTAX OPTIONS OPCODE OBJMODE RPT CYC
CMPB AX, #8bit
0101 001A CCCC CCCC
X − 1
Operands AX Accumulator high (AH) or accumulator low (AL) register
#8bit 8-bit immediate constant value
Description Compare the content of the specified AX register (AH or AL) with the
zero-extended 8-bit unsigned immediate constant. The result of (AX − 0:8bit)
is evaluated and the status flag bits are set accordingly. The content of the AX
register is left unchanged:
Set Flags On (AX − 0:8bit);
Flags
and
Modes
N If the result of the operation is negative, then N is set; otherwise it is cleared.
The CMPB instruction assumes infinite precision when it determines the sign
of the result. For example, consider the subtraction 0x8000 − 0x0001. If the
precision were limited to 16 bits, the result would cause an overflow to the
positive number 0x7FFF and N would be cleared. However, because the
CMPB instruction assumes infinite precision, it would set N to indicate that
0x8000 − 0x0001 actually results in a negative number.
Z The comparison is tested for a zero condition. The zero flag bit is set if the
operation (AX − [0:8bit]) = 0, otherwise it is cleared.
C If the subtraction generates a borrow, then C is cleared; otherwise C is set.
Repeat This instruction is not repeatable. If this instruction follows the RPT
instruction, it resets the repeat counter (RPTC) and executes only once.
Example
; Check if VarA is within range 0x80 <= VarA <= 0xF0:
MOV AL,@VarA ; Load AL with contents of VarA
CMPB AL,#0xF0 ; Set Flags On (AL − 0x00F0)
SB OutOfRange,GT ; Branch if VarA greater then 0x00FF
CMPB AL,#0x80 ; Set Flags On (AL − 0x0080)
SB OutOfRange,LT ; Branch if VarA less then 0x0080