Data Copy Between Z80 Registers and Memory

Data Copy Between Z80 Registers and Memory

To copy data from and into memory, the 16-bit address of a selected memory register must be specified, and this memory address can be specified by using indirect, direct, or other addressing modes in the indirect addressing mode, the address of a memory location is loaded into the HL register, and the register (HL) with the parentheses is used as a memory pointer to copy data. Registers BC and, DE can also be used in this manner, with some restrictions. In the direct addressing mode, the 16-bit address of a memory location is used as the operand of the copy instruction. Methods using index registers are-discussed after the discussion of 2’s complement arithmetic because the index registers include a displacement byte, which is expressed as a signed 2’s complement number. In Z80 mnemonics, the memory address is enclosed in parentheses, as shown in the following list.

General Characteristics

I. No flags are affected by these data copy operations.

2. Memory-related data copy operations can be recognized by the parentheses around the operand.

3. Register HL is a versatile memory pointer; a data byte can be copied from any memory location to any general-purpose register and vice versa. In addition, HL can be used to load a byte directly into memory.

4. A 16-bit direct address and other register pairs (BC and DE) can be used as memory pointers to copy data from a memory location into the accumulator and vice versa. However, these memory pointers cannot be used to copy data between general-purpose registers and memory.

Example

The memory location 2050_H contains the data byte 37_H. Write instructions to copy the byte from the memory location into register B. Illustrate three different ways of transferring the byte from memory to the microprocessor, and list the associ­ated machine codes.

Solution

1. The first method of copying a byte from memory into the microprocessor is by using the HL register as a memory pointer; this is an illustration of indirect addressing. First, we need to load the memory address into the HL register and then use the contents of HL as a memory pointer .

2. The second method of copying a byte from memory into the microprocessor is by using BC or DE as a memory pointer; this is also the indirect addressing . However, these registers (BC and DE) can be used as pointers to copy into A only. Therefore, one more instruction is necessary to copy from A into B.

3. The third technique is to use the direct extended addressing copies a data byte from memory into A and then into B.

Example

The memory location 2040_H contains the data byte F2_H. copy the data byte F2_H from the memory location 2040_H into 2070_H using memory pointers Then, clear the memory location 2040_H. Enter the machine codes of these instructions in memory locations starting from 2000_H Describe how data copy operations are performed.

Solution

Description

1. The first two instructions load registers HL and BC with the numbers 2040_H and 2070_H, respectively, These are not memory-related data copy instructions because the operands Jack parentheses.

2. The next two instructions copy the data byte (F2_H) stored in memory location 2040_H into the accumulator and from the accumulator into location 2070_H

3. The next instruction LD (HL), 00 is a 2-byte instruction; it clears the memory location 2040_H by loading 00 into the memory location pointed to by the H register.