Table B-2. 8051 Instruction Set
Instr.
Cycles
Hex
Code
Mnemonic
JNB bit, rel
Description
Byte
Jump on direct bit = 0
3
3
1
2
2
3
3
3
3
2
3
4
4
3
3
3
4
4
4
4
3
4
30
10
JBC bit, rel
Jump on direct bit = 1 and clear
Jump indirect relative DPTR
JMP @ A+DPTR
JZ rel
73
Jump on accumulator = 0
60
JNZ rel
Jump on accumulator /= 0
70
CJNE A, direct, rel
CJNE A, #d, rel
CJNE Rn, #d, rel
CJNE @ Ri, #d, rel
DJNZ Rn, rel
DJNZ direct, rel
Compare A, direct JNE relative
Compare A, immediate JNE relative
Compare reg, immediate JNE relative
Compare Ind, immediate JNE relative
Decrement register, JNZ relative
Decrement direct byte, JNZ relative
B5
B4
B8-BF
B6-B7
D8-DF
D5
Miscellaneous
NOP
No operation
1
1
00
There is an additional reserved opcode (A5) that performs the same function as NOP. All mnemonics
are copyrighted. Intel Corporation 1980.
B.1.3 Instruction Timing
Instruction cycles in the 8051 core are 4 clock cycles in length, as opposed to the 12 clock
cycles per instruction cycle in the standard 8051. This translates to a 3X improvement in
execution time for most instructions.
Some instructions require a different number of instruction cycles on the 8051 core than they
do on the standard 8051. In the standard 8051, all instructions except for MULand DIVtake
one or two instruction cycles to complete. In the 8051 core, instructions can take between one
and five instruction cycles to complete.
For example, in the standard 8051, the instructions MOVX A, @DPTRand MOV direct,
directeach take 2 instruction cycles (24 clock cycles) to execute. In the 8051 core, MOVX
A, @DPTRtakes two instruction cycles (8 clock cycles) and MOV direct, directtakes
three instruction cycles (12 clock cycles). Both instructions execute faster on the 8051 core
than they do on the standard 8051, but require different numbers of clock cycles.
For timing of real-time events, use the numbers of instruction cycles from Table B-1. to
calculate the timing of software loops. The bytes column indicates the number of memory
EZ-USB v1.9
Appendix B: 8051 Architectural Overview
B - 9
CYPRESS [ CYPRESS ]
