Sharp MZ-80B Owner's Manual page 79

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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 any one 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 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 by the programmer.
They also simplify programming, especially in ROM based systems where little external read/write memory is available.
1.2 ARITHMETIC AND LOGIC UNIT (ALU)
The 8-bit arithmetic and logical instructions of the CPU are executed in the ALU. Internally the ALU communi-
cates with the registers and the external data bus on the internal data bus. The type of 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
1.3 INSTRUCTION REGISTER AND CPU CONTROL
As each instruction is fetched from memory , it is placed in the instruction register and decoded . The control sec-
tion 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.