ZiLOG Z80 User Manual page 23

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The CPU includes two independent 8-bit accumulators and associated 8-
bit flag registers. The accumulator holds the results of 8-bit arithmetic or
logical operations while the register indicates specific conditions
FLAG
for 8-bit or 1 16-bit operations, such as indicating whether or not the
result of an operation is equal to zero. The programmer selects the
accumulator and flag pair with a single exchange instruction so that it is
possible to work with either pair.
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Two matched sets of general-purpose registers, each set containing six 8-bit
registers, may be used individually as 8-bit registers or as 16-bit register
pairs. One set is called BC, DE, and HL while the complementary set is
called BC', DE', and HL'. At any one time, the programmer can select either
set of registers to work through a single exchange command for the entire
set. In systems that require fast interrupt response, one set of general-purpose
registers and an register may be reserved for handling
ACCUMULATOR/FLAG
this fast routine. One exchange command is executed to switch routines.
This greatly reduces interrupt service time by eliminating the requirement
for saving and retrieving register contents in the external stack during
interrupt or subroutine processing. These general-purpose registers are used
for a wide range of applications. They also simplify programing, specifically
in ROM-based systems where little external read/write memory is available.
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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 by using the internal data bus. Functions performed by
the ALU include:
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