Handshake Solutions
HT80C51 User Manual
80C51 Family Instruction Set – Instruction definitions
Page 108 of 132
© Philips Electronics N.V. 2005
JMP @A+DPTR
Function: Jump indirect
Description: Add the eight-bit unsigned contents of the Accumulator with the sixteen-bit data pointer, and load
the resulting sum to the program counter. This will be the address for subsequent instruction
fetches.
Sixteen-bit addition is performed (modulo 216): a carry-out from the low-order eight bits propa-
gates through the higher-order bits. Neither the Accumulator nor the Data Pointer is altered. No
flags are affected.
Example: An even number from 0 to 6 is in the Accumulator. The following sequence of instructions will
branch to one of four AJMP instructions in a jump table starting at JMP_TBL:
MOV
DPTR,#JMP_TBL
JMP
@A+DPTR
JMP_TBL: AJMP LABEL0
AJMP LABEL1
AJMP LABEL2
AJMP LABEL3
If the Accumulator equals 04H when starting this sequence, execution will jump to label LABEL2.
Remember that AJMP is a two-byte instruction, so the jump instructions start at every other ad-
dress.
Bytes: 1
Cycles: 2
Encoding:
Operation: JMP
(PC)
← (A) + (DPTR)
JNB bit,rel
Function: Jump if Bit Not set
Description: If the indicated bit is a zero, branch to the indicated address; otherwise proceed with the next
instruction. The branch destination is computed by adding the signed relative-displacement in the
third instruction byte to the PC, after incrementing the PC to the first byte of the next instruction.
The bit tested is not modified. No flags are affected.
Example: The data present at input port 1 is 11001010B. The Accumulator holds 56H (01010110B). The
instruction sequence,
JNB P1.3,LABEL1
JNB ACC.3,LABEL2
will cause program execution to continue at the instruction at label LABEL2.
Bytes: 3
Cycles: 2
Encoding:
Operation: JNB
(PC)
← (PC) + 3
IF (bit) = 0
THEN
(PC)
← (PC) + rel
0
1
1
1
0
0
1
1
0
0
1
1
0
0
0
0
bit address
rel. address
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