Jump if not Carry (JNC) in 8085 Microprocessor

Microprocessor8085

In 8085 Instruction set,we are having one mnemonic JNC a16, which stands for “Jump if Not Carry” and “a16”stands for any 16-bit address. This instruction is used to jump to the address a16 as provided in the instruction. But as it is a conditional jump so it will happen if and only if the present carry flag value is 0. If carry flag value is 1, program flow continues sequentially. It is a 3-Byte instruction.

Mnemonics, Operand
Opcode(in HEX)
Bytes
JNCLabel
D2
3


Let us consider one example of this instruction type JNC 4000H. It is a 3-Byte instruction. The result of execution of this instruction is shown below with an example.

Address
Hex Codes
Mnemonic
Comment
2000
37
STC
Set Cy flag bit. So Cy = 1
2001
3F
CMC
Complement Cy flag bit. So Cy = 0
2002
D2
JNC 4000
Jump Not Carry, i.e. Jump when Cy = 0
2003
00


Low order Byte of the target address
2004
40


High order Byte of the target address
PC ← 4000H, So the program control will be transferred to the address 4000H
2005
78
MOV A, B
This instruction will not get control now as JNC will transfer the control to the memory address 4000H
….
….
….
….
4000
41
MOV B, C
Next instruction at address 4000H will get the control


The timing diagram against this instruction JNC 4000H execution is as follows –


Summary − So this instruction JNC requires 3-Bytes, 3-Machine Cycles (Opcode Fetch, Memory Read, MemoryRead) and 10 T-States for execution as shown in the timing diagram.


raja
Published on 27-Dec-2018 11:22:41
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