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PicoBlaze 8-bit Embedded Microcontroller www.xilinx.com 27
UG129 (v1.1.2) June 24, 2008
Processing Data
R
See also:
• “ADD sX, Operand —Add Operand to Register sX,” page 91
• “ADDCY sX, Operand —Add Operand to Register sX with Carry,” page 92
SUB and SUBCY Subtract Instructions
The PicoBlaze microcontroller provides two subtract instructions, SUB and SUBCY, that
compute the difference of two 8-bit operands, either without or with CARRY (borrow),
respectively. The CARRY flag indicates if the subtract operation generates a borrow
condition. The first operand is a register location. The second operand is either a register
location or a literal constant. The resulting operation affects both the CARRY and ZERO
flags. If the resulting difference is less than 0, then the CARRY flag is set. If the resulting
difference is 0 or -256, then the ZERO flag is set.
The SUBCY instruction is a subtract operation with borrow. If the CARRY flag is set, then
SUBCY subtracts an additional one from the resulting difference.
The SUBCY instruction is commonly used in multi-byte subtraction. Figure 3-9
demonstrates a subroutine that subtracts two 16-bit integers and produces a 16-bit
difference. The upper byte of each 16-bit value is labeled as MSB for most-significant byte;
the lower byte of each 16-bit value is labeled LSB for least-significant byte.
See also:
• “SUB sX, Operand —Subtract Operand from Register sX,” page 113
• “SUBCY sX, Operand —Subtract Operand from Register sX with Borrow,” page 114
Increment/Decrement
The PicoBlaze microcontroller does not have a dedicated increment or decrement
instruction. However, adding or subtracting one using the ADD or SUB instructions
provides the equivalent operation, as shown in Figure 3-10.
Figure 3-8: 16-Bit Addition Using ADD and ADDCY Instructions
ADD16:
NAMEREG s0, a_lsb ; rename register s0 as “a_lsb”
NAMEREG s1, a_msb ; rename register s1 as “a_msb”
NAMEREG s2, b_lsb ; rename register s2 as “b_lsb”
NAMEREG s3, b_msb ; rename register s3 as “b_lsb”
ADD a_lsb, b_lsb ; add LSBs, keep result in a_lsb
ADDCY a_msb, b_msb ; add MSBs, keep result in a_msb
RETURN
Figure 3-9: 16-Bit Subtraction Using SUB and SUBCY Instructions
SUB16:
NAMEREG s0, a_lsb ; rename register s0 as “a_lsb”
NAMEREG s1, a_msb ; rename register s1 as “a_msb”
NAMEREG s2, b_lsb ; rename register s2 as “b_lsb”
NAMEREG s3, b_msb ; rename register s3 as “b_lsb”
SUB a_lsb, b_lsb ; subtract LSBs, keep result in a_lsb
SUBCY a_msb, b_msb ; subtract MSBs, keep result in a_msb
RETURN