2. Programming Information
2.1 General
The MC-10 is built around two Motorola products: the MC6803 microprocessor,
and the MC6847 video generator. The original Colour Computer used the MC6847
for its display as well, but contained the somewhat more advanced, and
non-compatible, MC6809 processor.
The 6803 is the last chip in Motorola's 6800 line, and builds on the
original with the inclusion of 128 bytes of on-chip RAM, a hardware multiply
instruction, and a built-in serial port. The MC6803 is the precursor to
(I believe) the MC68HC11, which is still being manufactured.
Hitachi also marketed an "enhanced" clone of the MC6803, the HD6303,
as is used in the Psion PDA. Many people have improved the performance
of their Colour Computer by replacing the 6809 CPU with a Hitachi 6309,
which leads me to wonder if installing an HD6303 and custom ROM in an MC-10
would have the same effect. Also extant is the MC68HC16, a sixteen-bit
processor which is source-compatible with the MC6800.
2.2 Memory Map
Memory in the MC-10 is allocated as follows:
| $E000 |
- |
$FFFF: |
Internal ROM (BASIC) |
| $C000 |
- |
$DFFF: |
External expansion ROM |
| $5000 |
- |
$BFFF: |
External expansion RAM |
| $4200 |
- |
$4FFF: |
Internal RAM |
| $4000 |
- |
$41FF: |
Video memory |
| $0100 |
- |
$3FFF: |
Unused |
| $0080 |
- |
$00FF: |
MC6803 on-chip RAM |
| $0020 |
- |
$007F: |
Unused |
| $0000 |
- |
$001F: |
MC6803 internal registers |
Many thanks to Bob Devries of Australia for providing me with this correct
memory map of the MC-10, among other information.
The MC6803, like the original 6800, has 256 bytes of fast-access, "direct
page" memory from $0000 to $00FF. Operations which support the direct addressing
mode require only a one-byte operand to address this memory. The MC6803
also contains 128 bytes of on-chip RAM, from $0080 to $00FF; my guess is
that accessing this memory is very fast.
Display memory starts at $4000. The standard text display occupies the
first (32 x 16 =) 512 bytes of memory; this video mode also supports 64
x 32 "block" graphics. With an additional 16k of memory installed, higher-resolution
graphics modes of the MC6847 can be used as well.
Micro Colour Basic lives in the top eight kilobytes of memory, from
$E000 to $FFFF, and by default is mirrored immediately below at $C000.
External devices can override this mirroring and use memory addresses from
$C000 to $DFFF as their own. (Was Radio Shack planning an Extended Micro
Colour Basic cartridge?)
The last eight words of memory store interrupt vectors used by the 6803,
as follows:
| $FFFE: |
Reset vector |
| $FFFC: |
NMI non-maskable interrupt vector |
| $FFFA: |
SWI software interrupt vector |
| $FFF8: |
IRQ 1 interrupt vector |
| $FFF6: |
IRQ 2 timer input capture vector |
| $FFF4: |
IRQ 2 timer output compare vector |
| $FFF2: |
IRQ 2 timer overflow vector |
| $FFF0: |
IRQ 2 serial I/O interrupt vector |
In the MC-10 ROM, these vectors point to the following memory addresses,
in order: $4200, $4203, $4206, $4209, $420C, $420F, $4212, and $F72E. Note
that the first seven vectors point to addresses in RAM to allow software
to introduce their own interrupt handlers, although the unfortunate choice
of memory locations interferes with the high-resolution video modes (see
below). $F72E is the BASIC ROM's entry point.
2.3 I/O Devices
All I/O in the MC-10 is memory-mapped. As soon as I determine the memory
addresses used to communicate with the on-board serial port and cassette
interface, I will post them here.
2.3.3 Keyboard
The MC-10's forty-eight key, calculator-style keyboard connects to the
mainboard through two, eight-conductor ribbon cables. Keyboard status can
be obtained by reading from $BFFF in memory -- I'll post more information
here soon.
2.3.1 Serial Interface
The built-in, four-pin DIN, RS-232C serial port supports a maximum transfer
rate of 9600 baud, although it is limited to 600 baud operation (via the
LPRINT and LLIST commands) under BASIC. Pin assignments on the serial port
are as follows:
-
Carrier detect (CD)
-
Receive data (RD)
-
Ground (GND)
-
Transmit data (TD)
Micro Colour Basic sets the serial port to 600 baud, 1 start bit, 7 data
bits, 2 stop bits, and no parity. When time permits, I'll post instructions
on building a null modem cable to allow data transfer between the MC-10
and a PC.
2.3.2 Cassette Interface
The cassette interface operates at an average speed of 1600 baud, and the
pin assignments on the five-pin DIN connector are as follows:
-
Remote ground
-
Signal ground
-
Remote control
-
Audio input (earphone jack)
-
Audio output (aux-in or mic jack)
Note that regular Colour Computer and Model I/III/IV cassette interface
cables can be used with the MC-10. According to the MC-10 manual, the cassette
interface and RS-232C serial port can not be used simultaneously. Also,
remote control of the cassette recorder does not occur in Micro Colour
Basic (and is probably not supported by the hardware).
Data is stored serially on the cassette, lowest-order bit first. A binary
"one" is encoded as a complete 2400 Hz cycle, and a binary "zero" as a
complete 1200 Hz cycle. The MC-10 decodes the audio data by measuring the
time between zero-crossings of the input signal.
The Micro Colour BASIC file format, which is compatible with that used
by the Colour Computer, defines the notion of a block, which stores up
to 255 bytes of data together with header and checksum information. Three
types of blocks are recognized: filename; data; and end-of-file.
An Micro Colour BASIC saved file has the following structure:
-
A 128-byte leader
-
A filename block
-
A half-second, silent gap
-
Another 128-byte leader
-
One or more data blocks
-
An end-of-file block
Each leader consists of 128 bytes of hexadecimal 55, binary 01010101, and
is used by the MC-10 for synchronization.
Blocks have the following general format:
-
A two-byte ID string, $553C
-
A one-byte block type
-
A one-byte data length
-
Between 0 and 255 data bytes, as specified
-
A checksum byte
-
A one-byte end-of-block ID, $55
A block's checksum is supposedly calculated as the sum of its data, type
and length bytes, although I haven't verified this. Valid block types include
filename, $00; data, $01; and end-of-file, $FF. Filename blocks contain
fifteen bytes of data, structured as follows:
-
An eight-byte file name
-
A one-byte file type
-
A one-byte data type
-
A one-byte gap present flag
-
A two-byte start address
-
A two-byte load address
File types include BASIC programs, $00; data, $01; and machine code, $02.
The data type is either binary, $00; or ASCII, $FF. The gap-present flag
will be either $01, indicating no silent gaps between subsequent blocks;
or $FF, indicating that gaps will be present. The load and start addresses
for machine-language programs are stored in the last two words of the filename
block.
End-of-file blocks contain zero bytes of data and always appear at the
end of saved data.
I have written C routines to extract the data from WAV recordings of
MC-10 saved programs, and I will be making the source code, along with
a proposed, open file format for the archival of MC-10 software, available
from this website as soon as possible. (If you're interested, you can have
a look at the work in progress.) In the meantime,
it is possible to use Jeff Vavasour's (closed-source) Colour
Computer transfer utilities under DOS to archive MC-10 saved programs.
Information regarding the Micro Colour BASIC file format was acquired
from Bill Yakowenko's Color
Computer Stuff webpage and through experimentation with a real MC-10.
2.3.4 Video Generator
Video output is handled by the MC6847 and fed directly to the built-in
RF modulator. Composite output is not provided, but could be added assuming
the MC1372 RF modulator chip is used.
Operation of the MC6847 is controlled by writing to address $BFFF in
memory, where each bit corresponds -- almost -- to a control line on the
MC6847. The mapping is as follows:
| 7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
| Sound |
CSS |
A/G |
GM0 |
GM1 |
GM2,
INT/EXT |
N/C |
N/C |
Bit seven is not connected to the video generator at all. Rather, toggling
this bit produces sound from the TV speaker. Bits one and zero are also
not connected. The pin abbreviations are as follows:
-
CSS: Colour select. Toggles between colour palettes.
-
A/G: Alphanumerics/graphics (low/high) select.
-
INT/EXT: Selects internal (low) or external (high) character generator
ROM. Only the internal ROM is available on the MC-10; this line is provided
to toggle between semigraphics modes.
-
GM0-2: Graphics mode select lines.
To simplify things, the INT/EXT and GM2 lines are tied together on the
MC-10. Also, the A/S (alphanumerics/semigraphics) control line is pulled
high by the hardware, meaning that alphanumerics-only modes are inaccessible.
The following table lists the video modes available on the MC-10, and
the bit patterns required to activate them:
| A/G |
GM0 |
GM1 |
GM2 |
Video Mode |
Colours |
Memory |
| 0 |
X |
X |
0 |
Semigraphics 4 (SG4) |
8 |
512 bytes |
| 0 |
X |
X |
1 |
Semigraphics 6 (SG6) |
8 |
512 bytes |
| 1 |
0 |
0 |
0 |
64 x 64 Colour Graphics One (CG1) |
4 |
1024 bytes |
| 1 |
1 |
0 |
0 |
128 x 64 Resolution Graphics One (RG1) |
2 |
1024 bytes |
| 1 |
0 |
1 |
0 |
128 x 64 Colour Graphics Two (CG2) |
4 |
2048 bytes |
| 1 |
1 |
1 |
0 |
128 x 96 Resolution Graphics Two (RG2) |
2 |
1536 bytes |
| 1 |
0 |
0 |
1 |
128 x 96 Colour Graphics Three (CG3) |
4 |
3072 bytes |
| 1 |
1 |
0 |
1 |
128 x 192 Resolution Graphics Three (RG3) |
2 |
3072 bytes |
| 1 |
0 |
1 |
1 |
128 x 192 Colour Graphics Six (CG6) |
4 |
6144 bytes |
| 1 |
1 |
1 |
1 |
256 x 192 Resolution Graphics Six (RG6) |
2 |
6144 bytes |
Note that RG3 is the highest video mode usable on an unexpanded MC-10.
For all practical purposes, modes CG6 and RG6 require the 16k memory expansion.
Micro Colour Basic directly supports only the Semigraphics 4 video mode.
To access different video modes from BASIC, POKE the corresponding decimal
value from the following chart into memory address 49151:
| Mode |
Palette |
POKE Value |
|
Mode |
Palette |
POKE Value |
| SG4 |
Green |
0 |
$00 |
RG2 |
Green |
56 |
$38 |
| Orange |
64 |
$40 |
Buff |
120 |
$78 |
| SG6 |
Green |
4 |
$04 |
CG3 |
Green |
36 |
$24 |
| Orange |
68 |
$44 |
Buff |
100 |
$64 |
| CG1 |
Green |
32 |
$20 |
RG3 |
Green |
52 |
$34 |
| Buff |
96 |
$60 |
Buff |
116 |
$74 |
| RG1 |
Green |
48 |
$30 |
CG6 |
Green |
44 |
$2C |
| Buff |
112 |
$70 |
Buff |
108 |
$6C |
| CG2 |
Green |
40 |
$28 |
RG6 |
Green |
60 |
$3C |
| Buff |
104 |
$68 |
Buff |
124 |
$7C |
In semigraphics mode, the background colour of the screen can be toggled
between green (CSS = 0) and orange (CSS = 1). In the resolution graphics
modes, the drawing colour can be selected from green (CSS = 0) and buff
(off-white; CSS=1). In the four-colour graphics modes, colour palettes
are defined as in the chart below:
| Palette |
Value |
Colour |
| Green (CSS = 0) |
0 |
Green |
| 1 |
Yellow |
| 2 |
Blue |
| 3 |
Red |
| Buff (CSS = 1) |
0 |
Buff |
| 1 |
Cyan |
| 2 |
Magenta |
| 3 |
Orange |
2.3.5 Memory Expansion Connector
At the back of the MC-10, behind a plastic cover attached by screws, is
a 34-pin card edge connector which provides access to the address and data
busses of the MC6803. The pin assignment is shown below.
| Top |
Bottom |
| GND |
1 |
|
2 |
GND |
| D0 |
3 |
|
4 |
D1 |
| D2 |
5 |
|
6 |
D3 |
| D4 |
7 |
|
8 |
D5 |
| D6 |
9 |
|
10 |
D7 |
| R/-W |
11 |
|
12 |
A0 |
| A1 |
13 |
|
14 |
A2 |
| A3 |
15 |
|
16 |
A4 |
| A5 |
17 |
|
18 |
A6 |
| A7 |
19 |
|
20 |
A8 |
| A9 |
21 |
|
22 |
A10 |
| A11 |
23 |
|
24 |
A12 |
| A13 |
25 |
|
26 |
A14 |
| A15 |
27 |
|
28 |
E |
| SEL |
29 |
|
30 |
-RESET |
| -NMI |
31 |
|
32 |
+5V |
| GND |
33 |
|
34 |
GND |
A0-A15 and D0-D7 form the address and data busses, respectively. Active
low signals are prefixed with a hyphen. The R/W line indicates whether
the contents of the data bus are to be written or read. Non-maskable interrupts
are generated by a trailing edge on NMI; holding RESET low for three cycles
will restart the CPU. According to the MC6802 (not MC6803) datasheet, it
is safe to tie NMI and RESET directly to +5 volts if they are not used.
I also gather from the MC6802 datasheet that the clock input to the CPU
is duplicated at E.
The memory expansion connector was, not surprisingly, generally used
to attach Radio Shack's 16k memory expansion. No other peripherals which
used this port were marketed in North America. Apparently in France, a
robotic arm accessory was sold for the Alice computers; I would imagine
that it connected to this port -- verification, anyone?
2.3.6 Other
There is one CPU-dependent sound channel, which is also fed to the RF modulator
and output from the television speaker. Sound is produced by toggling bit
seven of memory address $BFFF (49151). The BASIC SOUND command can also
be used to produce tones of varying pitch and duration.
2.4 Programming Tools
The MC-10 can be programmed using the BASIC interpreter in ROM, which provides
most of the commands available in the original Colour Basic. There are
also a couple of new commands added, and even an undocumented command:
"EXEC," which allows the user to transfer control to a machine-language
routine in memory.
Any assembler or compiler which outputs machine code for the MC6800,
6801, or 6803 processor can be used to produce machine-language software
for the MC-10. (Remember, the MC6809 used in the Colour Computer is a very
different processor!)
Thanks to all who wrote to tell me that CLOADM and EXEC can be used
to load and execute machine-language programs. (I still haven't been able
to verify that CLOADM is recognized by Micro Colour BASIC, however.)
I am currently working on a small, MC6803 development suite for the
MC-10, written in Java and C, although I've made little progress in the
past couple of months. You can still download the Java-based disassembler
in zip (Windows 95/NT, OS/2) or tar-gzip
format (Linux and other UNIXes). I'll eventually get around to adding links
to other freely-available MC6800 development tools as well.
The Radio Shack MC-10 microcomputer is the lesser-known, younger cousin of the Tandy Colour Computer. Small enough to sit on the palm of your hand, the MC-10 came equipped with four kilobytes of RAM, a Motorola MC6803 eight-bit microprocessor, a built-in serial port, and the same graphics capabilities as the original Colour Computer. Underpowered for its time, the MC-10 was released in 1983 but discontinued in 1984, along with the 16k memory upgrade and small amount of cassette-based software which had been released for it.
