Relative Jump Instructions

Relative Jump Instructions

The Z80 instruction set includes two types of relative Jump instructions: uncon­ditional and conditional. The new address to which the program sequence is re­directed is specified by an 8-bit offset (displacement) value relative to the Jump instruction. The displacement can be positive (forward jump), specified by the seven bits D₆-D₀ (the MSB D₇ = 0), or negative (backward jump) specified in 2’s complement. The total offset values range from -126 to + 129 bytes. (explained in Example). The list of relative Jump instructions is (d = displacement).

General Characteristics

1. These are 2-byte instructions; therefore, they are more efficient than 3-byte absolute jump instructions in terms of memory space and, in some situations, execution time.

2. Relative jumps are limited to 256 memory locations.

3. No flags are affected by these instructions.

Example

The unconditional relative Jump instruction is stored in memory locations 2100 and 2101_H, as shown below. Find the memory address of the forward jump loca­tion if the displacement byte is 7F_H. and find the memory address for the back­ward jump if the displacement byte is 9C_H.

Solution

1. When the jump instruction is executed, the program counter (PC) contains the .address 2102(PC always points to the next machine code to be fetched). By adding the displacement byte to the program counter, the address of the jump location becomes 2181_H (2102_H + 7F_H).

For an 8-bit displacement byte, 7F_H is the largest offset value for a for­ward jump. Therefore, relative to the memory location of the first code of the Jump instruction, the maximum displacement is 7F_H plus two memory loca­tions of the instruction. The decimal equivalent of 81_H (7F + 2) is 129; thus, the positive range extends to 129 memory locations.

2. If the displacement byte is 9C_H, it is in 2’s complement because D₇ = l. The memory address for the backward jump location from 2l02_H (contents of the program counter) can be calculated two ways: (a) by adding the negative dis­placement 9C_H or (b) by subtracting the positive displacement 64_H, which is the 2’s c9mplement of 9C_H. ·

20 9E

The memory address of the Jump location is 209E_H. In the first calculation, after adding a negative number, the sum is 9E_H without a carry. Therefore, in 2’s com­plement addition, the ninth bit must be set to l, indicating a borrow that is subtracted from 21_H. If you are unfamiliar with 2’s complement arithmetic, review section B.2 in Appendix B. In the second calculation, 64_H is a positive number obtained by taking 2’s complement of the negative number, and it is subtracted from 2102_H.