Fix typo in goat.asm comment

.editorconfig

@@ -3,6 +3,8 @@ root = true
 [*]
 charset = utf-8
 insert_final_newline = true
+end_of_line = lf
+tab_width = 4
 
 [*.asm]
 indent_style = tab
@@ -11,12 +13,10 @@ trim_trailing_whitespace = true
 [*.{masm.asm,nasm.asm}]
 charset = latin1
 end_of_line = crlf
-tab_width = 4
 
 [*.gas.asm]
-end_of_line = lf
 tab_width = 8
 
 [*.py]
-end_of_line = lf
-tab_width = 4
+indent_style = tab
+trim_trailing_whitespace = true

real/goat.asm

@@ -0,0 +1,24 @@
+; 100-byte COM sacrificial goat executable (1993, author unknown)
+; Binary MD5: 195307045CC39D6B284B60442ECFD202
+;     SHA256: D1F60FCA64F1903F8D405109C5AA55A3F3B6DDE622BCFBA15CD95001CAE1DEE2
+;
+; Assemble with FASM: fasm goat.asm goat.com
+; Assemble with NASM or YASM: nasm -fbin goat.asm -o goat.com
+
+			use16
+			org 100h
+
+start:
+			jmp		short print
+			nop
+
+hello_str	db 'Hello - This is a 100   COM test file, 1993', 0Ah, 0Dh, '$'  ; Hello followed by '\n\r'
+
+			db 1Ah						; Pad with substitute
+			times 41 db 'A'				;  and 'A' * 41
+
+print:
+			mov		ah, 9				; AH: Print string
+			mov		dx, hello_str		; DS:DX: String = "Hello - This is a 100   COM test file, 1993"
+			int		21h
+			int		20h					; Return to DOS

technomid.py

@@ -3,11 +3,12 @@
 # Home page: https://github.com/ScrelliCopter/TECHNO.COM
 # SPDX-License-Identifier: Zlib  (https://opensource.org/license/Zlib)
 
+import os
 import struct
 import math
 from abc import ABC, abstractmethod
 from pathlib import Path
-from typing import BinaryIO, List, Tuple
+from typing import BinaryIO, Iterable, Tuple
 from enum import IntEnum
 
 
@@ -54,7 +55,7 @@ class MIDIProgramChange(MIDIEvent):
 		if channel < 0 or channel > 0xF:
 			raise ValueError("MIDI Channel out of range")
 		if patch < 0 or patch >= 0x80:
-			raise ValueError("Program out of range")
+			raise ValueError("MIDI Program out of range")
 		self._channel = channel
 		self._patch = patch
 
@@ -75,6 +76,11 @@ class MIDIPitchWheel(MIDIEvent):
 		return struct.pack(">BBB", 0xE0 | self._channel, self._value & 0x7F, self._value >> 7)
 
 
+class MIDIMetaTrackEnd(MIDIEvent):
+	def serialise(self) -> bytes:
+		return struct.pack(">BBB", 0xFF, 0x2F, 0x00)
+
+
 class MIDIMetaTempo(MIDIEvent):
 	def __init__(self, quarter_us: int):
 		if quarter_us < 0 or quarter_us >= 0x1000000:
@@ -89,80 +95,111 @@ class MIDIWriter:
 	def __init__(self, file: BinaryIO):
 		self._file = file
 
-	Format = IntEnum("Format", ["SINGLE", "MULTI", "SEQUENCE"])
+	class Format(IntEnum):
+		SINGLE   = 0
+		MULTI    = 1
+		SEQUENCE = 2
 
 	def write_header(self, division: int, fmt: Format = Format.SINGLE, track_count: int = 1):
 		self._file.write(b"MThd")
 		self._file.write(struct.pack(">IHHH", 6, fmt, track_count, division))
 
-	def write_track(self, events: List[Tuple[int, MIDIEvent]]):
-		payload = b""
-		for event in events:
-			payload += self.encode_varint(event[0])
-			payload += event[1].serialise()
+	def write_track(self, events: Iterable[Tuple[int, MIDIEvent]]):
 		self._file.write(b"MTrk")
-		self._file.write(len(payload).to_bytes(4, byteorder="big"))
-		self._file.write(payload)
+		ofs = self._file.tell()
+		self._file.write(b"\0\0\0\0")  # Blank length field to write later
 
+		# Serialise and write out events
+		payload_len = 0
+		for event in events:
+			data = event[1].serialise()
+			length = self.encode_varint(event[0])
+			self._file.writelines([length, data])
+			payload_len += len(length) + len(data)
+
+		# Fill in track length field
+		self._file.seek(ofs)
+		self._file.write(payload_len.to_bytes(4, byteorder="big"))
+		self._file.seek(0, os.SEEK_END)
+
+	# Variable integer, used by event deltas.
+	# Up to 4 bytes can encode 7 bits each by setting the continuation bit (bit 8)
 	def encode_varint(self, value: int) -> bytes:
-		if value < 0:
-			raise ValueError("Variable integer must be positive")
-		if value >= 0x10000000:
-			raise ValueError("Variable integer is larger than 0FFFFFFF")
 		if value < 0x80:
-			return value.to_bytes()
+			return value.to_bytes(1, byteorder="big")
+		if value < 0x4000:
+			return bytes([0x80 | (value >> 7), value & 0x7F])
+		if value < 0x200000:
+			return bytes([0x80 | (value >> 14), 0x80 | (value >> 7) & 0x7F, value & 0x7F])
+		if value < 0x10000000:
+			return bytes([0x80 | (value >> 21), 0x80 | (value >> 14) & 0x7F, 0x80 | (value >> 7) & 0x7F, value & 0x7F])
 		else:
-			a = (value & 0xFE00000) >> 21
-			b = (value & 0x01FC000) >> 14
-			c = (value & 0x0003F80) >> 7
-			d = (value & 0x000007F)
-			if a != 0:
-				return bytes([a | 0x80, b | 0x80, c | 0x80, d])
-			elif b != 0:
-				return bytes([b | 0x80, c | 0x80, d])
-			elif c != 0:
-				return bytes([c | 0x80, d])
+			raise ValueError("Variable integer out of range")
 
 
-def generate(outpath: Path):
-	timer = round((1000000 * 14.31818) / 12)
+def generate(f: BinaryIO):
+	def note_from_period(period: int, reference: int) -> (int, int):
+		frequency = reference / max(1, period)        # Convert period to hz
+		fnote = 69 + 12 * math.log2(frequency / 440)  # Convert pitch to MIDI note
+		note = int(round(fnote))                      # Snap to nearest semitone
+		bend = int(round((fnote - note) * 0x1000))    # Error is encoded as pitch bend
+		return min(0x7F, note), min(0x1FFF, bend)
 
-	def note_from_period(period: int) -> (int, int):
-		frequency = timer / max(1, period)
-		fnote = 69 + 12 * math.log2(frequency / 440)
-		note = int(round(fnote))
-		bend = min(0x1FFF, int(round((fnote - note) * 0x1000)))
-		return min(0x7F, note), bend
+	def parse_midi_note(note: str) -> int:
+		flat_sharp = 0
+		octave = 3
+		if len(note) > 1:
+			if note[1] == '-' or note[1].isnumeric():
+				octave = int(note[1:])
+			else:
+				flat_sharp = { "b": -1, "#": 1 }[note[1]]
+				if len(note) > 2:
+					octave = int(note[2:])
 
-	def techno():
+		natural = { "C": 0, "D": 2, "E": 4, "F": 5, "G": 7, "A": 9, "B": 11 }
+		return natural[note[0]] + flat_sharp + (1 + octave) * 12
+
+	def techno(length: int):
+		timer = int(round((1000000 * 1260 / 88) / 12))  # Intel 8253 (PIC) clock in Mhz
+
+		# Music tables from disassembly
 		phrase = [2, *[1, 0, 0] * 3] * 3 + [2, 3] + [0, 3] * 3
-		freq_tbl = [5424, 2712, 2416, 2280]
+		note_tbl = ["A3", "A4", "B4", "C5"]
 		mangler = 0x0404
+		mangler_inc = 76  # len(phrase) * 2
+
+		# Convert note table to period
+		freq_from_note = lambda m: 440.0 * 2.0 ** ((m - 69) / 12.0)
+		period_from_freq = lambda f: int(round(timer / f))
+		period_from_note = lambda s: period_from_freq(freq_from_note(parse_midi_note(s)))
+		freq_tbl = [period_from_note(note) for note in note_tbl]
+
+		yield 0, MIDIMetaTempo(int(round((1000000 * 0x80000) / timer)))  # 16th note every two PIC ticks
+		yield 0, MIDIProgramChange(0, 80)  # Set GM patch to #81 "Lead 1 (Square)"
 
 		i = 0
 		bend = 0
 		while True:
 			for sixteenth in phrase:
-				note, new_bend = note_from_period(freq_tbl[sixteenth])
+				note, new_bend = note_from_period(freq_tbl[sixteenth], timer)
 				if new_bend != bend:
 					yield 0, MIDIPitchWheel(0, new_bend)
 					bend = new_bend
 				yield 0, MIDINoteOn(0, note)
 				yield 32, MIDINoteOff(0, note)
 				i += 2
-				if i >= 80 * 25:
+				if i >= length:
+					yield 0, MIDIMetaTrackEnd()
 					return
-			mangler = (mangler + len(phrase) * 2) & 0xFFFF
+			# Scramble pitch table at the end of each measure
+			mangler = (mangler + mangler_inc) & 0xFFFF
 			freq_tbl = [freq ^ mangler for freq in freq_tbl]
 
-	with outpath.open("wb") as f:
-		mid = MIDIWriter(f)
-		mid.write_header(128)
-		mid.write_track([
-			(0, MIDIProgramChange(0, 80)),
-			(0, MIDIMetaTempo(int(round((1000000 * 0x80000) / timer))))
-		] + list(techno()))
+	mid = MIDIWriter(f)
+	mid.write_header(128)
+	mid.write_track(techno(80 * 25))
 
 
 if __name__ == "__main__":
-	generate(Path("techno.mid"))
+	with Path("techno.mid").open("wb") as f:
+		generate(f)