Xilinx UG129 manual Shift and Rotate Instructions

R

Chapter 3: PicoBlaze Instruction Set

The example in Figure 3-25demonstrates how to generate parity for all eight bits in a register.

generate_parity:

TEST sX, FF ; include all bits in parity generation

Figure 3-25:Generate Parity for a Register Using the TEST Instruction

See also “TEST sX, Operand — Test Bit Location in Register sX, Generate Odd Parity,” page 116.

Compare

The COMPARE instruction performs an 8-bit subtraction of two operands but only affects the ZERO and CARRY flags, as shown in Table 3-3. No registers are modified.

The ZERO flag is set when both input operands are identical. When set, the CARRY flag indicates that the second operand is greater than the first operand.

Table 3-3:COMPARE Instruction Flag Operations

Flag	When Flag=0	When Flag=1
ZERO	Operand_1 ≠ Operand_2	Operand_1 = Operand_2
CARRY	Operand_1 > Operand_2	Operand_1 < Operand_2

See also “COMPARE sX, Operand — Compare Operand with Register sX,” page 96.

Shift and Rotate Instructions

Shift

The PicoBlaze microcontroller supports a rich set of shift instructions, summarized in

Table 3-4, that modify the contents of a single register. All shift instructions affect the

CARRY and ZERO flags.

The SL0 sX instruction shift the contents of register sX left by one bit position. The most- significant bit, bit 7, shifts into the CARRY flag. The least-significant bit position is filled with a ‘0’. The SR0 instruction is similar except the least-significant bit, bit 0, shifts into the CARRY flag and the most-significant bit is filled with a ‘0’.

The SL1 and SR1 shift instructions are similar to SL0 and SR0 except that the empty bit location is filled with a ‘1’. The ZERO flag is always ‘0’ when using SL1 and SR1 because there is always a ‘1’ shifted into the affected register, making the register non-zero.

The SRX sX instruction performs an arithmetic shift right operation and sign extends register sX, preserving the sign bit. The most-significant bit, bit 7, is unaffected during the shift operation and is copied back into bit 7. The SLX sX instruction is similar but shifts the register sX contents to the left, replicating bit 0 and filling the register with the bit 0 value.

The SLA sX instruction left shifts the contents of register sX through the CARRY bit, the CARRY bit feeding back into the least-significant bit, bit 0, of register sX. The SRA sX instruction is similar to SLA but with a right shift.

34	www.xilinx.com	PicoBlaze 8-bit Embedded Microcontroller
		UG129 (v1.1.2) June 24, 2008