Page 50 - DCAP210_INTRODUCTION__TO_MICROPROCESSORS

Introduction to Microprocessors

Notes 3.3.4.2 Conditional Jump Instructions

Conditional jumps allow the processor to make decisions on certain conditions which are indicated
by the flags. After every logic and arithmetic operation, flags are set or reset to reflect the status of
the result. The conditional jump instructions check the flag status and make decisions about the
sequence of the program. The 8085 has five flags, out of which auxiliary carry flag (AC) is used
internally. The other four flags are used by the jump instructions. They are:
(a) Carry flag (CY)

(b) Zero flag (Z)
(c) Sign flag (S)
(d) Parity flag (P).
Two jump instructions are associated with each flag. The sequence of a program can be changed
either – because the condition is present or the condition is absent. All conditional jump instructions
are 3-byte instructions. The first byte is the opcode, the second byte specifies the low-order memory
address and the third byte specifies the high-order memory address. The high-order memory
address denotes page number and low-order memory address denotes the line number. The
conditional jump instructions are stated in Table 3.6.

Table 3.6: List of Conditional Jump Instructions

Opcode Operand Description Remarks

JC 16-bit Jump on Carry If result generates carryand CY = 1.
JNC 16-bit Jump on No Carry When CY = 0
JZ 16-bit Jump on Zero If result is zero and Z = 1

JNZ 16-bit Jump on No Zero When Z = 0
JP 16-bit Jump on Plus If (MSB) D = 0 and S = 0
7
JM 16-bit Jump on Minus If (MSB) D = 1 and S = 1
7
JPE 16-bit Jump on Even Parity When P = 1

JPO 16-bit Jump on Odd Parity When P = 0

Conditional jumps are an important aspect of the decision-making process in
the programming. These instructions test for a certain condition (e.g. Zero or
Carry flag) and alter the program sequence when the condition is met. In
addition, the instruction set includes an instruction called unconditional jump.

3.3.5 Call, Return, and Restart
These instructions change the sequence of a program either by calling a subroutine or returning
from a subroutine. The conditional Call and Return instructions also can test condition flags.

3.3.6 Machine Control Operations
These instructions control machine functions such as Halt, Interrupt, or do nothing.

The microprocessor operations related to data manipulation can be summarized in four functions:
1. Copying data
2. Performing arithmetic operations

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