shithub: plan9front

ref: b4d4cf69be84b92796b5e5bd81f16999a54bff39

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- 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