ref: 357e58699e6ababa2f971461a4918f552bca1e9e
dir: /sys/src/games/blit/mmap/
- main CPU is a MC68000 running at 8 MHz.
the exact clock speed is most likely one of: 7.9872, 7.68 or 7.3728 MHz (to divide to 19200 baud)
- display is 800x1024, refreshed at 30 Hz (60 Hz interlaced).
it simply displays the 100K framebuffer from the address given in register 030.
- the host is connected through a UART running at 19200 baud.
- the keyboard is connected through a UART running at an unknown baudrate.
- the mouse is connected through some unknown circuitry.
memory map
start end purpose
0x00000 0x00008 see register 050 below
0x00008 0x40000 RAM (256K)
0x40000 ??? ROM (24K ?) (not known if mirrored)
0x60000 ??? memory mapped I/O
mmio addresses (in *octal*):
0000 16-bit mouse y
0002 16-bit mouse x
0011 8-bit terminal UART status/control (MC 6850 ACIA)
0013 8-bit terminal UART data
the software uses 2 stop bits, no parity, 8 bits; clock divided by 16
0021 8-bit mouse buttons
bit 0: rmb
bit 1: mmb
bit 2: lmb
reading clears the mouse interrupt
0025 8-bit second copy of 0021 (?)
0027 8-bit ???
only the ROM binaries contain references to 0025, 0027 in buttons.c.
the corresponding source code appears to have been deleted.
possibly some older variant of the button hardware?
0030 16-bit start address of framebuffer divided by 4 (800*1024/8 = 100K bytes)
0040 16-bit status/control register of display
bit 0: invert display
with this bit set 0 bits are white and 1 bits are black (most software does this).
0050 16-bit writing different values makes 0-8 either mapped to 0x40000 or error out (details unclear)
resets so that they map to the reset vector at 0x40000.
0056 8-bit some kind of sound-making device (standard programmable timer + piezoelectric speaker?)
0156 referenced by demo pacman.
0256
0060 8-bit keyboard status/control register (MC 6850 ACIA)
the software uses 2 stop bits, no parity, 8 bits; clock divided by 16
0062 8-bit keyboard data register
0070 16-bit? acknowledge vblank interrupt; software writes 0
tx to the keyboard sets the 7 lights and can also ring the bell.
could deduce the order of lights from diag.c.
writing 2 is used to sound the bell.
irq lines; using the 68k autovectoring
vector 1 (0x64) is vblank
vector 2 (0x68) is keyboard
vector 4 (0x70) is mouse buttons
vector 5 (0x74) is uart
MC 6850 bits:
+0 (r) status register
bit 0: receive buffer full
bit 1: transmit buffer empty
bit 2: data carrier detected (inverted)
bit 3: clear-to-send (inverted)
bit 4: framing error
bit 5: receiver overrun
bit 6: parity error
bit 7: irq active
+0 (w) control register
bit 1-0: clock divider
00: /1
01: /16
10: /64
11: master reset
bit 4-2: parity and stop bits (see datasheet)
bit 6-5:
RTS/ is high only if these bits are 10
TX interrupts are enabled only if these bits are 01
break is transmitted continuously if these bits are 11
bit 7: RX interrupt enabled