The 6846 is the main PIA chip in all machines for the TO range. It handles the tape drive, memory banks commutations, 50Hz interrupt (actually programmable, it's generated using the timer), and a few extra things.

Register map

E7C0 CSR composite status register
E7C1 CRC Port C control
E7C2 DDRCPort C direction
E7C3 PRC Port C data
E7C4 composite status register
E7C5 TCR timer control
E7C6 TMSBtimer most significant byte
E7C7 TLSBtimer least significant byte



P0: Video RAM mapping

This bit allows switching the video RAM page for the CPU. when set to 0, the “color” (attributes) page is mapped. When set to 1, the “shape” (pixels) page is mapped.

(on TO8/9+/8D, this is not actually wired, but the Gate Array emulates it).

P1: light pen button

This bit is an input. It allows you to read the state of the light pen “click” button.

P3: unused.

P7: Tape drive input

The data from the tape drive can be read here. The tape drive use hardware demodulation, so what you read here is to be stored directly to memory. There is no need for software decoding. This bit can also be used to detect the tape drive, it is at fixed 1 when the tape drive is plugged and no data is found, but 0 when the tape drive is not connected.

CP2: Sound output

This is the 1-bit sound output. Setting it to 0 and 1 alternatively, at the right speed, may generate squarewaves.

CTO: Timer output to tape port

The timer is used as a waveform generator, and the output is sent to the tape drive to save data to tape. As the tape port also has an audio input pin, connecting the two would allow to use the waveform generator for low-CPU sound and music. This has not been done, yet.

The tape format for TO machines is:

  • To code a 1, generate 5 periods of 4.5KHz squarewave
  • To code a 0, generate 7 periods of 6.3KHz squarewave.

The decoding is done using analogue filters set on these frequencies. A comparator between the filters output decides if the bit is a 0 or a 1.

Note this is the same timer that generates the cursor blink interrupt (at a 10Hz frequency).


P2 and P6: "RAMDisk" expansion control

These two bits allows mapping of the “RAM Disk” memory (external expansion on the TO9). When no internal RAM is selected (bits PB3-PB7 from System 6821 PIA are all 1), the value of these two bits define the mapped RAM disk page.

P6P2PB7PB6PB5PB4PB3Mapped memory at A000-DFFF
X X 1 1 1 X X TO7 32K expansion mappings
X X 1 1 1 1 0 Internal bank 0
X X 1 1 1 0 1 Internal bank 1
X X X X 0 1 1 TO9 extra internal RAM mappings
X X 0 0 0 1 1 Internal bank 2
X X 1 0 0 1 1 Internal bank 3
X X 0 1 0 1 1 Internal bank 4
X X 1 1 0 1 1 Internal bank 5
X X 1 1 1 1 1 TO9 external RAM mappings
0 0 1 1 1 1 1 RAMdisk 1
1 0 1 1 1 1 1 RAMdisk 2
0 1 1 1 1 1 1 RAMdisk 3
1 1 1 1 1 1 1 RAMdisk 4

P4 and P5: Cartridge slot selection

On TO9 and later, these two bits are used to switch between different mappings for the cartridge area (0000-3FFF). It allows software (un)mapping of cartridges.

P4P5Bank 0 Bank 1 Bank 2 Bank 3
0 0 BASIC 128 Extramon BASIC 1.0 Icon DOS
0 1 Paragraphe
1 0 Fiches et Dossiers
1 1 External cartridge

Each slot may have up to 4 banks, switching by the banks is done by writing anywhere between 0000 and 1FFF. The two low-order address bits define the bank number.

  • Writing to 0000 selects bank 0
  • Writing to 0001 selects bank 1
  • Writing to 1F01 selects bank 1
  • Writing to 1E02 selects bank 2

Monitor entry EC03 (COMSLOT) allows calling code from one ROM bank to another.


PB2: Cartridge switch

Only 2 banks are available.

PB4: Monitor bank switch

also switches the floppy ROM banks.

Reading keyboard state

The keyboard link was implemented on the TO9 using a 6850 ACIA. To save costs, it is now implemented using a bit-bang serial link driven by the PIA.

When the keyboard controller want to send a byte, it raises CP1 high. The computer acknowledges by setting PB5 low.

The serial frame is made of:

  • CNT key state
  • SHIFT key state
  • 7-bit keycode of first pressed key (unlike on TO9, this is not the ASCII code).

The signalling is:

  • 56µs high pulse for 1 bit
  • 38µs high pulse for 0 bit

Configuring the keyboard

The computer can send commands by pulling PB5 low, waiting for CP1 to become low (acknowledge), then leave PB5 low for the right amount of time.

  • 670µS: keyboard reset. The keyboard enter caps lock mode (led is on), and send the keymap bit (French or Export) to the computer.
  • 1300µS: enable caps lock
  • 1900µS: disable caps lock

PB6: printer busy bit

documentations/devices/6846.txt · Last modified: 2015/02/28 18:08 (external edit)
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