Implement prelude-loading mechanism

README.md

@@ -46,3 +46,12 @@ registers: `xBUF`, `xHEAD` and `xTAIL`:
 In order to be distinguishable from the empty state, the ring buffers
 must never be completely full -- there must always be *at least one*
 unoccupied byte between the tail and the head.
+
+# System status
+
+| Name     | Offset | Size / B | Access       |
+|----------|--------|----------|--------------|
+| SYSREADY | 68     | 1        | atomic write |
+
+The `SYSREADY` register is used to indicate when the system has booted
+up and is ready for user input.

emu.js

@@ -4,10 +4,11 @@ const TXHEAD = 64;
 const TXTAIL = 65;
 const RXHEAD = 66;
 const RXTAIL = 67;
+const SYSREADY = 68;
 
 const TXBUF_SIZE = 32;
 const RXBUF_SIZE = 32;
-const PERIPHS_SIZE = 68;
+const PERIPHS_SIZE = 69; // Nice
 
 const POLL_INTERVAL_MS = 20;
 
@@ -44,13 +45,18 @@ class Emulator {
 			end: { x: 0, y: 0 }
 		};
 		this.idle_timer = null;
+		this.input_enable = false;
 		document.addEventListener('keydown', (e) => this.handle_keydown(e));
 
-		this.flush_output();
-		document.getElementById('cursor').classList.add('blinking');
-
 		this.worker = new Worker('boot.js');
 		this.worker.postMessage(this.mem);
+
+		fetch('prelude.f')
+			.then(res => res.text())
+			.then(text => {
+				for (const cu of this.encoder.encode(text))
+					this.rx_queue.push(cu);
+			});
 	}
 
 	poll() {
@@ -65,6 +71,15 @@ class Emulator {
 			if (this.fifo_next(rxtail) != rxhead)
 				this.handle_rx_data(rxhead, rxtail);
 		}
+
+		if (!this.input_enable) {
+			const sysready = Atomics.load(this.mem_u8, SYSREADY);
+			if (sysready != 0) {
+				this.input_enable = true;
+				this.flush_output();
+				document.getElementById('cursor').classList.add('blinking');
+			}
+		}
 	}
 
 	fifo_next(idx) {
@@ -118,6 +133,9 @@ class Emulator {
 	}
 
 	handle_keydown(e) {
+		if (!this.input_enable)
+			return;
+
 		if (e.key.length == 1 && !e.ctrlKey && !e.altKey && !e.metaKey) {
 			this.shift_up(this.cursor);
 			this.grid[this.cursor.y][this.cursor.x] = e.key;
@@ -239,7 +257,8 @@ class Emulator {
 		const html = this.grid.map((row, y) => {
 			return row.map((c, x) => {
 				const ec = this.html_escape(c);
-				if (x == this.cursor.x && y == this.cursor.y)
+				if (this.input_enable
+					&& x == this.cursor.x && y == this.cursor.y)
 					return '<span id="cursor">' + ec + '</span>';
 				else
 					return ec;

prelude.f

@@ -0,0 +1 @@
+1 68 AC!