8086 Program control instructions

1. Microprocessor & its Applications
Module 3 Continued……
Dr. Girisha G S
Dept. of CSE
SoE, DSU, Bengaluru
1

2. Agenda
• Program Control Instructions
• Jump Group
- unconditional jump
- conditional jump

3. Program Control Instructions
- Program control instructions change or modify the flow of a program
Jump Group of instructions
Jump instructions are classified in to two groups
1. Unconditional Jump
- Control is transferred unconditionally to the
target label which points to the address of target
instruction
E.g. Jmp, call, ret
2. Conditional Jump
- Control is transferred to the target label if a certain
condition is met, otherwise execution continues
with next sequential instruction
- The condition are represented by flags
E.g. jc, jz, jnc, jnz

4. JMP - Unconditional Jump
• Three types : short jump, near jump, and far jump.
• The short jump is a two-byte instruction that allows jumps or branches
to memory locations within +127 and -128 bytes from the address
following the jump.
• A near jump is a three-byte instruction that allows a branch or jump
within ±32K bytes from the instruction in the current code segment.
• far jump is a 5 byte instruction that allows a jump to any memory
location within the real memory system.
• The short and near jumps are often called intrasegment jumps
• the far jumps are often called intersegment jumps.
The general form of the JMP instruction is
jmp label
where label is a program address indentifier
• This instruction unconditionally transfer control of the execution to the specified label
(address) using an 8 bit or 16 bit displacement or CS:IP.
• No flags are affected by this instruction.
• When this instruction is executed ,the CS & IP are unconditionally modified to a new
value of CS & IP corresponding to the location transferred

5. Short Jump
• Short jumps are called relative jumps because they can be moved to any
location in current code segment without a change
• A short jump to four
memory locations
beyond the address of
the next instruction.
• The short jump instruction appears in figure below.
• Example shows how short jump instructions
pass control from one part of the program
to another.
Example
jmp short next (use short directive to force short jump).

6. Near Jump
• The near jump is similar to the short jump, except that the distance is
farther.
• A near jump passes control to an instruction in the current code segment
located within ±32K bytes from the near jump instruction.
A near jump that adds the
displacement (0002H) to
contents of IP.
• The near jump is a 3-byte instruction that
contains an opcode followed by a signed
16-bit displacement.
E.g. jmp next

7. Far Jump
• A far jump instruction transfer control to the target location in different
code segment. This is performed by loading CS with segment address in
which the target exist & IP with the offset address of a instruction to
which control is to be transferred in the new code segment.
• The target address loaded in CS & IP are part of the instruction itself. This is a
5 byte instruction.
A far jump instruction replaces the contents of
both CS and IP with 4 bytes following the
opcode.
E.g. jmp far ptr skip

8. Conditional Jumps Instructions
• The conditional jump instruction transfer control to the address specified (target location)
if some condition is met or satisfied otherwise execution continues sequentially.
• The condition here means the status of the condition flags. The conditional jump
instructions test the following flag bits: sign(S), zero(Z), carry (C), parity(P), overflow(O).
Instruction Condition Tested Comment
JA C = 0 and Z = 0 Jump above
JAE C = 0 Jump above or equal to
JB C = 1 Jump below
JBE C = 1 or Z = 1 Jump below or equal to
JC C=1 Jump carry set
JE or JZ Z = 1 Jump equal to or jump zero
JG Z = 0 and S = 0 Jump greater than
JGE S = 0 Jump greater than or equal to
JL S ≠ 0 Jump less than
JLE Z = 1 or S ≠ 0 Jump less than or equal to
JNC C = 0 Jump carry cleared
JNE or JNZ Z = 0 Jump not equal to or jump not zero
JNO 0 = 0 Jump no overflow
JNS S = 0 Jump no sign
JNP/JPO P = 0 Jump no parity/jump parity odd
JO 0 = 1 Jump on overflow
JP/JPE P= 1 Jump on parity/jump parity even
JS S = 1 Jump on sign
If the condition under test is true, a branch to the label associated with the jump
instruction occurs..
If the condition is false, the next sequential instruction in the program executes.
For example, a JC will jump if the carry bit is set.