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// Code ported from Beebem (C to .js) by Jason Robson
const TELETEXT_IRQ = 5;
const TELETEXT_FRAME_SIZE = 860;
const TELETEXT_UPDATE_FREQ = 50000;
/*
Offset Description Access
+00 Status register R/W
+01 Row register
+02 Data register
+03 Clear status register
Status register:
Read
Bits Function
0-3 Link settings
4 FSYN (Latches high on Field sync)
5 DEW (Data entry window)
6 DOR (Latches INT on end of DEW)
7 INT (latches high on end of DEW)
Write
Bits Function
0-1 Channel select
2 Teletext Enable
3 Enable Interrupts
4 Enable AFC (and mystery links A)
5 Mystery links B
*/
/**
* Emulates the Acorn teletext adaptor. Dispatches a `notice` CustomEvent, carrying a
* `message` in its detail, when a channel's stream cannot be loaded.
*/
export class TeletextAdaptor extends EventTarget {
constructor(cpu) {
super();
this.cpu = cpu;
// Not cleared by a reset, so a fetch still in flight across one is recognised as stale.
this.streamRequest = 0;
this.clearState();
}
clearState() {
this.teletextStatus = 0x0f; /* low nibble comes from LK4-7 and mystery links which are left floating */
this.teletextInts = false;
this.teletextEnable = false;
this.channel = 0;
this.currentFrame = 0;
this.totalFrames = 0;
this.rowPtr = 0x00;
this.colPtr = 0x00;
this.streamData = null;
this.pollCount = 0;
this.frameBuffer = new Array(16).fill(0).map(() => new Array(64).fill(0));
// Only a register access clears our IRQ, so an interrupt latched before the reset would hang the machine.
this.cpu.interrupt &= ~(1 << TELETEXT_IRQ);
}
reset(hard) {
if (!hard) return;
this.clearState();
this.loadChannelStream(this.channel);
}
async loadChannelStream(channel) {
console.log("Teletext adaptor: switching to channel " + channel);
const request = ++this.streamRequest;
let data;
try {
data = await loadData(`teletext/txt${channel}.dat`);
} catch (error) {
if (request !== this.streamRequest) return;
console.error(`Teletext adaptor: failed to load channel ${channel}`, error);
this.dispatchEvent(
new CustomEvent("notice", {
detail: {
message: `Teletext channel ${channel} could not be loaded (${error?.message ?? error}). The adaptor carries on with nothing to show.`,
},
}),
);
return;
}
// Fetches can resolve out of order; only the newest request may apply its data.
if (request !== this.streamRequest) return;
this.streamData = data;
this.totalFrames = Math.floor(data.length / TELETEXT_FRAME_SIZE);
this.currentFrame = 0;
}
updateIrq() {
if (this.teletextInts && this.teletextStatus & 0x80) {
this.cpu.interrupt |= 1 << TELETEXT_IRQ;
} else {
this.cpu.interrupt &= ~(1 << TELETEXT_IRQ);
}
}
snapshotState() {
return {
teletextStatus: this.teletextStatus,
teletextInts: this.teletextInts,
teletextEnable: this.teletextEnable,
channel: this.channel,
currentFrame: this.currentFrame,
rowPtr: this.rowPtr,
colPtr: this.colPtr,
pollCount: this.pollCount,
frameBuffer: this.frameBuffer.map((row) => row.slice()),
};
}
restoreState(state) {
this.teletextStatus = state.teletextStatus;
this.teletextInts = state.teletextInts;
this.teletextEnable = state.teletextEnable;
this.currentFrame = state.currentFrame;
this.rowPtr = state.rowPtr;
this.colPtr = state.colPtr;
this.pollCount = state.pollCount;
this.frameBuffer = state.frameBuffer.map((row) => row.slice());
this.updateIrq();
// Refetching the multi-megabyte stream on every restore would be ruinous for rewind.
if (this.channel !== state.channel) {
this.channel = state.channel;
this.loadChannelStream(this.channel);
}
}
read(addr) {
let data = 0x00;
switch (addr) {
case 0x00: // Status Register
data = this.teletextStatus;
break;
case 0x01: // Row Register
break;
case 0x02: // Data Register
data = this.frameBuffer[this.rowPtr][this.colPtr++];
break;
case 0x03:
this.teletextStatus &= ~0xd0; // Clear INT, DOR, and FSYN latches
this.cpu.interrupt &= ~(1 << TELETEXT_IRQ);
break;
}
return data;
}
write(addr, value) {
switch (addr) {
case 0x00:
// Status register
this.teletextInts = (value & 0x08) === 0x08;
this.updateIrq();
this.teletextEnable = (value & 0x04) === 0x04;
if ((value & 0x03) !== this.channel && this.teletextEnable) {
this.channel = value & 0x03;
this.loadChannelStream(this.channel);
}
break;
case 0x01:
this.rowPtr = value;
this.colPtr = 0x00;
break;
case 0x02:
this.frameBuffer[this.rowPtr][this.colPtr++] = value & 0xff;
break;
case 0x03:
this.teletextStatus &= ~0xd0; // Clear INT, DOR, and FSYN latches
this.cpu.interrupt &= ~(1 << TELETEXT_IRQ); // Clear interrupt
break;
}
}
// Attempt to emulate the TV broadcast
polltime(cycles) {
this.pollCount += cycles;
if (this.pollCount > TELETEXT_UPDATE_FREQ) {
this.pollCount = 0;
// Don't flood the processor with teletext interrupts during a reset
if (this.cpu.resetLine) {
this.update();
} else {
// Grace period before we start up again
this.pollCount = -TELETEXT_UPDATE_FREQ * 10;
}
}
}
update() {
if (this.currentFrame >= this.totalFrames) {
this.currentFrame = 0;
}
const offset = this.currentFrame * TELETEXT_FRAME_SIZE + 3 * 43;
this.teletextStatus &= 0x0f;
this.teletextStatus |= 0xd0; // data ready so latch INT, DOR, and FSYN
// The stream arrives asynchronously, so software can enable us before there is anything to copy.
if (this.teletextEnable && this.streamData) {
// Copy current stream position into the frame buffer
for (let i = 0; i < 16; ++i) {
if (this.streamData[offset + i * 43] !== 0) {
this.frameBuffer[i][0] = 0x67;
for (let j = 1; j <= 42; j++) {
this.frameBuffer[i][j] = this.streamData[offset + (i * 43 + (j - 1))];
}
} else {
this.frameBuffer[i][0] = 0x00;
}
}
}
this.currentFrame++;
this.rowPtr = 0x00;
this.colPtr = 0x00;
if (this.teletextInts) {
this.cpu.interrupt |= 1 << TELETEXT_IRQ;
}
}
}
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