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Return if parity even (RPE) in 8085 Microprocessor

In 8085 Instruction set, RPE is a mnemonic, which stands for “Return if Parity Even”. This instruction is used to return to the main program, only if P flag value is 1. If the P flag value is 0, program flow continues in the subroutine sequentially. It is a 1-Byte instruction.

Mnemonics, Operand
Opcode(in HEX)
Bytes
RPE
E8
1


Let us consider the following sample code for a better explanation –

Address
Hex Codes
Mnemonic
Comment
2000
31
LXI SP, 5000H
SP ← 5000H.Initializing the SP
2001
00


Low order Byte of the address
2002
50


High order Byte of the address
2003
21
LXI H, 4050H
HL ← 4050H, Initializing the HL register pair
2004
50


Low order Byte of the address
2005
40


High order Byte of the address
2006
CD
CALL 200BH
Calling the subroutine at address 200BH. So now the control of the program will be transferred to the location 200BH. And the return address 2009H i.e. address of the next instruction will be pushed on the top of the stack. As a result 4FFFH (SP – 1) will contain 20H and 4FFEH (SP – 2) will contain 09H respectively.
2007
0B


Low order Byte of the address
2008
20


High order Byte of the address
2009
77
MOV M, A
M ← A, Content of the Accumulator will be transferred to the memory location 4050H as it is pointed by HL register pair. So at 4050H memory location Accumulators content, 00H will be stored. After successful execution of the RPE instruction, control will come back to this instruction.
200A
76
HLT
End of the program.
200B
3E
MVI A, 40H
A ← 40H, Initializing the Accumulator with initial value 40H
200C
40


40H as operand
200D
06
MVI B, 40H
B ← 40H, Initializing the Register B with initial value 30H
200E
40


40H as operand
200F
90
SUB B
A ← A – B = 40H – 40H = 00H, As the computed result is zero, so P = 1
2010
E8
RPE
Return the control to the address 2009H. Return address 2009H will be popped out from the top of the stack.   So from address 4FFEH, 09H will be popped and from address 4FFFH, 20H will be popped and SP will get the initial address 5000H back as its content accordingly.
2011
80
ADD B
A ← A + B ← 00H + 40H = 40H.
(But in this example this line is unreachable, so will not get executed)
2012
77
MOV M, A
M ← A, Content of the Accumulator will be transferred to the memory location 4050H as it is pointed by HL register pair. So at 4050H memory location Accumulators content 40H will be stored.
(But in this example this line is unreachable, so will not get executed)
2013
C9
RET
Return the control to the address 2009H. Return address 2009H will be popped out from the top of the stack.  So from address 4FFEH, 09H will be popped and from address 4FFFH 20H will be popped and SP will get the initial address 5000H back as its content accordingly.
(But in this example this line is unreachable, so will not get executed)


The timing diagram against this instruction RPE execution is as follows –

Summary − So this instruction RPE requires 1-Byte, 3-Machine Cycles (Opcode Fetch, Memory Read, Memory Read) and 12 T-States for execution as shown in the timing diagram.

Updated on 27-Jun-2020 14:17:04

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