72
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
fa
st 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-
ti
on
performs this function and then generates and supplies all
of
the control signals necessary
to
re
ad or write data
from or to the registers, control the
ALU and provide all required external control signals.
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