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PicoBlaze 8-bit Embedded Microcontroller www.xilinx.com 31
UG129 (v1.1.2) June 24, 2008
Processing Data
R
No Operation (NOP)
The PicoBlaze instruction set does not have a specific NOP instruction. Typically, a NOP
instruction is completely benign, does not affect register contents or flags, and performs no
operation other than requiring an instruction cycle to execute. A NOP instruction is
therefore sometimes useful to balance code trees for more predictable execution timing.
There are a few possible implementations of an equivalent NOP operation, as shown in
Figure 3-18 and Figure 3-19. Loading a register with itself does not affect the register value
or the status flags.
Figure 3-17: 8-bit Divided by 8-bit Routine
; Divide Routine (8-bit / 8-bit = 8-bit result, remainder)
; ==================================================
; Shift and subtract algorithm
;
div_8by8:
NAMEREG s0, dividend ; preserved
NAMEREG s1, divisor ; preserved
NAMEREG s2, quotient ; preserved
NAMEREG s3, remainder ; modified
NAMEREG s4, bit_mask ; used to test bits in dividend,
; one-hot encoded, modified
;
LOAD remainder, 00 ; clear remainder
LOAD bit_mask, 80 ; start with most-significant bit (msb)
div_loop:
TEST dividend, bit_mask ; test bit, set CARRY if bit is '1'
SLA remainder ; shift CARRY into lsb of remainder
SL0 quotient ; shift quotient left (multiply by 2)
;
COMPARE remainder, divisor; is remainder > divisor?
JUMP C, no_sub ; if divisor is greater, continue to next bit
SUB remainder, divisor ; if remainder > divisor, then subtract
ADD quotient, 01 ; add one to quotient
no_sub:
SR0 bit_mask ; shift to examine next bit position
JUMP NZ, div_loop ; if bit_mask=0, then all bits examined
; otherwise, examine next bit
RETURN
T
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Figure 3-18: Loading a Register with Itself Acts as a NOP Instruction
nop:
LOAD sX, sX