Table of Contents
Advertisement
180
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.
Advertisement
Table of Contents
