ZiLOG Z80-CPU Technical Manual page 11

General Purpose Registers
There are two matched sets of general purpose registers, each set containing six 8-bit registers that
may be used individually as 8-bit registers or as 16-bit register pairs by the programmer. One set is called
BC, DE and HL while the complementary set is called BC', DE' and HL'. At anyone time the programmer
can select either set of registers to work with through a single exchange command for the entire set. In
systems where fast interrupt response is required, one set of general purpose registers and an accumulator/
flag register may be reserved for handling this very fast routine. Only a simple exchange commands need be
executed to go between the routines. This greatly reduces interrupt service time by eliminating the require-
ment for saving and retrieving register contents in the external stack during interrupt or subroutine process-
ing. These general purpose registers are used for a wide range of applications by the programmer. They also
simplify programming, especially in ROM based systems where little external read/write memory is
available.
2.2 ARITHMETIC & LOGIC UNIT (ALU)
The 8-bit arithmetic and logical instructions of the CPU are executed in the ALU. Internally the ALU
communicates with the registers and the external data bus on the internal data bus. The type of functions
performed by the ALU include:
Add Left or right shifts or rotates (arithmetic and logical)
Subtract
Logical AND
Logical OR
Logical Exclusive OR
Compare
Increment
Decrement
Set bit
Reset bit
Test bit
2.3 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 all of the control signals necessary
to read or write data from or to the registers, control the ALU and provide all required external control
signals.
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