Radio Shack TRS-80 Service Manual page 30

5-Meg Hard Disk
Service Manual
Program data
is
input
to the 8X300
(U30) on the Instruction
Data bus
(
IA0
-
IA130)
which
is
capable
of directly
accessing 8K words of program storage
;
however, the
controller uses only the first
ten address lines, IA0
through
IA9,
limiting onboard program storage
to IK words
.
Fast
10
Select
An extension byte has been added onto the instruction data
memory
to
provide port access decoding on
an
instruction-by-instruction basis
.
This "Fast
10 Select" byte
is
not processed by the 8X300, but
it is
decoded by
auxiliary hardware
(U39, U44, U45) to
provide eight read
strobes and eight write strobes which route data
to the
various devices distributed along the interface vector bus
.
The Fast
10
byte
is
latched into
a
6-bit latch
(U39)
trailing edge
of MCLK
to
ensure that
the data remains stable
during the entire instruction* This data selects
a
read
strobe and eight write strobes which route data
to the
various devices distributed along the interface vector bus
.
The Fast
10
byte
is
latched into
a
6-bit latch
(U39)
trailing edge
of MCLK
to
ensure that
the
data remains stable
during the entire instructions
.
This data selects
a
read
strobe and
a
write strobe through two l-of-8 decoders
(U44
and
U45) which are alternately enabled by the
WC*
control
strobe produced by
the 8X300* THe read strobe decoder
(U45)
is
always disqualified
at the
end of instructions by
MCLK'
(MCLK prime),
a
delayed copy
of MCLK, to
provide edges
on
read strobes during sequential read operations from various
ports. This delay compensates for timing races through the
Fast
10
latch
(U39)
and the control signals*
Because each decoder
has
a
unique
input,
it is
possible
to
select any read port with any write port during each
instruction* Data
is
transferred between the processor and
its
ports
on
a
separate 8-bit bus called the
10 bus* This
bus
is
active
low* It
must be noted that this bus
is in no
way related
to
the instruction data bus and should be
thought of as simply
an
8-bit bidirectional
10
bus
of the
8X300. In fact,
it
has been renamed
as 100
-
107 to reflect
this distinction*
Internal
Bus Control
Several bus control signals are produced by the 8X300
to
identify and strobe the data on the
10 bus*
Write Control
28
-