Jump and CALL Instructions

Jump and CALL Instructions

Jump and CALL instructions change the flow of the program by changing the contents of program counter. A jump permanently changes the program flow whereas call temporarily changes the program flow to allow another part of the program to run.

There are jump instructions which change the program flow if certain condition exists. For example, CJNE (compare and jump if not equal). This instruction compares the magnitude of the first two operands, and changes program flow if their values are not equal.

The following types of instructions change the program flow :

■ Jump on bit conditions.

■ Compare byte and jump if not equal.

■ Decrement byte and jump if zero.

■ Jump unconditionally.

■ Call a subroutine.

■ Return from a subroutine.

1. Jump and Call Program Range

We know that a jump and call instructions replace the contents of the program counter with a new program address. The new address can be specified either by specifying the difference between the new address and the current program counter contents or by specifying the entire new address. The difference, in bytes, of the new address from the address in the program counter is called the range of the jump or call. For example, if a jump instruction is located at program address 0200H, and the jump causes the program counter to become 0230H, then the range of the jump is 30H bytes. Jump and CALL instructions may have one of the three ranges.

■ Relative (short) range : +127 to - 128 (+7FH to - 80H)

■ Absolute range : 0000H to 07FFH

■ Long range : 0000H to FFFFH

2. Jump

The Table 15.8.1 shows the list of jump instructions supported by 8051.

3. CALL and Subroutines

There are two subroutine-call instructions. LCALL (Long Call) and ACALL (Absolute Call). Each increments the PC to the first byte of the following instruction, then pushes it onto the stack (low byte first). Saving both bytes increment the stack pointer by two. The subroutine's starting address is encoded in the same ways as LJMP and AJMP. The generic form of the call operation is the mnemonic CALL, which 8051 will translate into LCALL or ACALL as appropriate.

The return instruction RET pops the high and low-order bytes of the program counter successively from the stack, decrementing the stack pointer by two. Program execution continues at the address previously pushed : The first byte of the instruction immediately following the call.

Review Questions

1. What are the uses of LCALL and LJUMP instructions of 8051 ?

2. Explain about the instruction DJNZ.