Color Computer - Tandy 26-3334 Service Manual

Memory Management Unit
(MMU)
XFFAO
- XFFAF,
6
bits (Write only)
The 8-bit CPU
in the Color Computer
3
can directly address
only 64K bytes of
memory with
its
16
address lines
(AO
-
A15). The memory management
unit (MMU)
extends
the
address lines
to
19
(AO
-
A18). This allows
the computer
to
address
up
to
512K bytes
of
memory ($00000 -
$7FFFF).
The
MMU consists
of a
multiplexer
and
a 16 x
6-bit
of
RAM
array. Each
of the
6-bit elements
in this
RAM array
is
an
MMU task register, and
the task
registers are used
by the computer
to
determine
the proper 8K segment of memory
to
address. These registers
are divided into
2
sets,
8
registers
per set. The
TR bit
of
FF91 (task register select bit) determines which set
is
selected.
The
relationship between
the
data
in the task register and the generated
address
is
as follows:
Bit
Corresponding
memory
address
D5 D4
D3 D2
Dl DO
A18
AI7 A16 A15 A14
A13
DO-D5
(CPU
DATA)
AI3-AI5
TR
bit
AO-A3
AI3-AI8
(Extended addresses)
When
the
CPU needs
to
access memory outside
the standard I/O and control range
(XFF00
- XFFFF), CPU address lines A13
-
A15 and
the
TR
bit determine
the
address
of the task register which
the
MMU will access while SELECT
is low.
When
the
CPU writes data
to the
MMU, A0 - A3 determine
the
address of
the
task
register
to be
written
to
when SELECT
goes high.
-15-