Zilog Z08470 - Manual (Page 16)

Architectural Overview UM008007-0715
4
Z80 CPU
User Manual
isters are used for a wide range of applications. They also simplify programing, specifi-
cally in ROM-based systems in which little external read/write memory is available.
Arithmetic Logic Unit
The 8-bit arithmetic and logical instructions of the CPU are executed in the Arithmetic
Logic Unit (ALU). Internally, the ALU communicates with the registers and the external
data bus by using the internal data bus. Functions performed by the ALU include:
• Add
• Subtract
• Logical AND
• Logical OR
• Logical exclusive OR
• Compare
• Left or right shifts or rotates (arithmetic and logical)
• Increment
• Decrement
• Set bit
• Reset bit
• Test bit
Instruction Register and CPU Control
As each instruction is fetched from memory, it is placed in the Instruction Register and
decoded. The control sections performs this function and then generates and supplies the
control signals necessary to read or write data from or to the registers, control the ALU,
and provide required external control signals.
Pin Description
The Z80 CPU I/O pins are shown in Figure 3. The function of each pin is described in the
section that follows.