1. THE BASIC COMPUTER
The basic VZ-200 computer employs a Z-80A microprocessor (U4)
running at a clock speed of 3.58MHz. Two 8K x 8-bit
mask-programmed ROMs (U9,U10) contain the Microsoft BASIC
interpreter, while three 8K x 8-bit static RAMs (U2',U3' and
U4') provide program memory.
A 6847P-1 video controller chip (U15) and a further 2K x 8-bit
static RAM (U7) form the heart of the computer's video section.
These are coupled to the processor data bus via an octal
bidirectional buffer, U14.
A simple software scanning scheme is used for the keyboard. The
keys are arranged in eight rows, each of which can be pulled
down to low logic level by diodes connected to the eight least
significant address lines (AO-A7). The other sides of the keys
are connected to six column lines, which are connected to six of
the inputs of a gated octal buffer (U12), and also to six pullup
resistors. The outputs of the six corresponding outputs of U12
connect to processor data lines DO-D5.
Cassette input is handled by a simple one-transistor circuit,
together with one of the remaining elements of U12 (D6).
Cassette output is taken from Ul, an 8-bit latch, via outputs Ql
and Q2. Other outputs from this latch are used to operate the
internal piezo speaker (Q0 and Q5, used in push-pull), and to
control the mode (Q3) and background color (Q4) inputs of the
video controller chip.
Simplified decoding is used for selection of the various I/O
devices in memory space. The decoding is performed by U2 and U3.
The memory address ranges (in hex) occupied are as follows:
0000
- 1FFF:
2000 -
3FFF:
4000 -
67FF:
6800 - 6FFF:
7000 -
77FF:
7800 - 8FFF:
9000 -
FFFF:
BASIC ROM 0
BASIC ROM 1
Not occupied in basic unit
(Used by ROM/game cartridges)
Keyboard matrix + cassette input (Read)
Cassette output, speaker, VDC control (Write)
Video RAM
Inbuilt program RAM
Not occupied in basic unit
(Used by expansion RAM modules)
Note that due to the simplified addressing, latch Ul serving the
cassette output, speaker and video display controller
effectively occupies all addresses from 6800-6FFF inclusive.
Similarly the keyboard/cassette input buffer U12 also occupies
all of this address range, although the individual rows of keys
effectively occupy discrete addresses.
A memory map showing both hex and decimal address is shown
overleaf.
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