Press n or j to go to the next uncovered block, b, p or k for the previous block.
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// BBC BASIC's workspace, the same in BASIC I, II and IV: https://8bs.com/basic/memory.htm
const Lomem = 0x00;
const VariablesEnd = 0x02;
const Himem = 0x06;
const TextPointerOffset = 0x0a;
const TextPointerBase = 0x0b;
const Top = 0x12;
const PageHigh = 0x18;
const ResidentIntegers = 0x400;
const ResidentIntegerCount = 27;
const FirstVariableInitial = 0x40;
const LastVariableInitial = 0x7a;
const ProcedureChain = 0x4f6;
const FunctionChain = 0x4f8;
/** Where the OS notes the language ROM's number, in the Advanced User Guide's map of page &02. */
export const LanguageRomNumber = 0x28c;
const RomTitle = 9;
const RomBase = 0x8000;
const FirstToken = 0x80;
const LineNumberToken = 0x8d;
const Return = 0x0d;
const Quote = 0x22;
const Colon = 0x3a;
const Star = 0x2a;
const RemToken = 0xf4;
const DataToken = 0xdc;
const Space = 0x20;
const LastPrintable = 0x7e;
const EndOfProgram = 0x80;
const LineHeaderBytes = 4;
const IntegerBytes = 4;
const ExponentBias = 128;
const MantissaSign = 0x80;
const SignificantDigits = 9;
const LargestFixedExponent = 8;
const SmallestFixedExponent = -1;
/** More lines than fit in memory, so that a walk over memory that is not a program ends. */
const MostLines = 0x4000;
/**
* Every BASIC jsbeeb carries reads each statement's first non-space character with `LDA (&0B),Y`, skipping spaces with
* `CMP #&20: BEQ`, then tells a keyword from an assignment with `CMP #&CF`, the first statement token. The search finds
* that `CMP`, run once for every statement.
*/
const StatementDispatch = [0xb1, 0x0b, 0xc9, 0x20, 0xf0, null, 0xc9, 0xcf];
const DispatchCompareOffset = 6;
/** The keyword table starts with AND, its first token. */
const FirstKeyword = [0x41, 0x4e, 0x44, FirstToken];
const KeywordCharacters = /^[A-Z][A-Z$(]*$/;
function findBytes(rom, pattern) {
const found = [];
for (let at = 0; at + pattern.length <= rom.length; ++at)
if (pattern.every((byte, i) => byte === null || rom[at + i] === byte)) found.push(at);
return found;
}
/** Each token's keyword, from the ROM's own table of keywords, each followed by its token and a byte of flags. */
function keywordsIn(rom) {
const keywords = new Map();
let [at] = findBytes(rom, FirstKeyword);
Iif (at === undefined) return keywords;
for (;;) {
let end = at;
while (end < rom.length && rom[end] < FirstToken) ++end;
const keyword = String.fromCharCode(...rom.subarray(at, end));
if (end >= rom.length || !KeywordCharacters.test(keyword)) return keywords;
if (!keywords.has(rom[end])) keywords.set(rom[end], keyword);
at = end + 2;
}
}
/**
* What a language ROM shows of BASIC: where it starts each statement, and its keywords; null for a ROM that is not a
* BASIC this can follow.
* @param {Uint8Array} rom
* @returns {{statementStart: number, keywords: Map<number, string>} | null}
*/
export function basicRom(rom) {
const dispatches = findBytes(rom, StatementDispatch);
if (dispatches.length !== 1) return null;
return { statementStart: RomBase + dispatches[0] + DispatchCompareOffset, keywords: keywordsIn(rom) };
}
/** A sideways ROM's title, from its header. */
export function romTitle(rom) {
const end = rom.indexOf(0, RomTitle);
return String.fromCharCode(...rom.subarray(RomTitle, end < 0 ? RomTitle : end)).replace(/[^ -~]/g, "");
}
const word = (peek, address) => peek(address) | (peek(address + 1) << 8);
const int32 = (peek, address) => word(peek, address) | (word(peek, address + 2) << 16);
function real(peek, address) {
const exponent = peek(address);
Eif (exponent === 0) return 0;
const top = peek(address + 1);
const mantissa =
((top | MantissaSign) * 2 ** 24 + (peek(address + 2) << 16) + (peek(address + 3) << 8) + peek(address + 4)) /
2 ** 32;
return (top & MantissaSign ? -1 : 1) * mantissa * 2 ** (exponent - ExponentBias);
}
/** A number as PRINT shows it with the default @%: nine significant figures, beyond which, or below 0.1, in E form. */
export function formatReal(value) {
if (value === 0) return "0";
const sign = value < 0 ? "-" : "";
const [mantissa, exponentText] = Math.abs(value)
.toExponential(SignificantDigits - 1)
.split("e");
const exponent = Number(exponentText);
const digits = mantissa.replace(".", "").replace(/0+$/, "");
if (exponent > LargestFixedExponent || exponent < SmallestFixedExponent) {
const fraction = digits.length > 1 ? `.${digits.slice(1)}` : "";
return `${sign}${digits[0]}${fraction}E${exponent}`;
}
if (exponent === SmallestFixedExponent) return `${sign}0.${digits}`;
const whole = digits.slice(0, exponent + 1).padEnd(exponent + 1, "0");
const fraction = digits.slice(exponent + 1);
return `${sign}${whole}${fraction ? `.${fraction}` : ""}`;
}
function printable(peek, from, length) {
let text = "";
for (let i = 0; i < length; ++i) {
const code = peek((from + i) & 0xffff);
text += code >= Space && code <= LastPrintable ? String.fromCharCode(code) : ".";
}
return text;
}
function stringValue(peek, address) {
return `"${printable(peek, word(peek, address), peek(address + 3))}"`;
}
/** An array's name with its bounds, as DIM gave them: the stored sizes are each one more. */
function arrayName(peek, name, address) {
const dimensions = (peek(address) - 1) / 2;
const bounds = [];
for (let i = 0; i < dimensions; ++i) bounds.push(word(peek, address + 1 + 2 * i) - 1);
return `${name}${bounds.join(",")})`;
}
/**
* The entries of one of BASIC's chains: a link to the next, then the rest of the name, ended by a zero. BASIC ends a
* chain with a link whose high byte is zero, and leaves the low byte as it was. Every entry is in the heap, between
* LOMEM and the end of the variables, and in no other chain, so a walk over memory that is not BASIC's ends there.
* @param {(address: number) => number} peek
* @param {number} head
* @param {{lomem: number, end: number, seen: Set<number>}} heap
*/
function* chain(peek, head, heap) {
let entry = word(peek, head);
while (entry >> 8 !== 0 && entry >= heap.lomem && entry < heap.end && !heap.seen.has(entry)) {
heap.seen.add(entry);
let name = "";
let at = entry + 2;
for (let code = peek(at); code !== 0 && at < heap.end; code = peek(++at)) name += String.fromCharCode(code);
yield { name, value: (at + 1) & 0xffff };
entry = word(peek, entry);
}
}
function variable(peek, name, address) {
Iif (name.endsWith("(")) return { name: arrayName(peek, name, address), value: "(array)" };
if (name.endsWith("%")) return { name, value: String(int32(peek, address)) };
Iif (name.endsWith("$")) return { name, value: stringValue(peek, address) };
return { name, value: formatReal(real(peek, address)) };
}
/**
* BASIC's variables, from its workspace: its dynamic variables in order of name, and its procedures and functions.
* @param {(address: number) => number} peek
*/
export function readVariables(peek) {
const heap = { lomem: word(peek, Lomem), end: word(peek, VariablesEnd), seen: new Set() };
const variables = [];
for (let initial = FirstVariableInitial; initial <= LastVariableInitial; ++initial) {
const first = String.fromCharCode(initial);
for (const { name, value } of chain(peek, ResidentIntegers + 2 * initial, heap))
variables.push(variable(peek, first + name, value));
}
variables.sort((a, b) => (a.name < b.name ? -1 : a.name > b.name ? 1 : 0));
const named = (prefix, head) => [...chain(peek, head, heap)].map(({ name }) => prefix + name);
return { variables, routines: [...named("PROC", ProcedureChain), ...named("FN", FunctionChain)] };
}
/**
* The resident integers, @% (the print format, in hex) and A% to Z%.
* @returns {{name: string, value: string}[]}
*/
export function readResidentIntegers(peek) {
return Array.from({ length: ResidentIntegerCount }, (_, i) => {
const value = int32(peek, ResidentIntegers + IntegerBytes * i);
const name = `${String.fromCharCode(FirstVariableInitial + i)}%`;
return i === 0
? {
name,
value: `&${(value >>> 0)
.toString(16)
.toUpperCase()
.padStart(2 * IntegerBytes, "0")}`,
}
: { name, value: String(value) };
});
}
/** Where BASIC's program and variables are, and the address it is reading the program at. */
export function readPointers(peek) {
return {
page: peek(PageHigh) << 8,
top: word(peek, Top),
lomem: word(peek, Lomem),
himem: word(peek, Himem),
/** The character last read: in a statement, the statement's, and at the start of one, its first. */
reading: (word(peek, TextPointerBase) + peek(TextPointerOffset) - 1) & 0xffff,
};
}
/**
* @typedef {object} ProgramLine
* @property {number} number
* @property {number} start the address of the return before it
* @property {number} end the address of the return after it
*/
/**
* The program's lines from PAGE: each a return, the line number high byte first, its length counting those four bytes,
* then its text.
* @returns {Generator<ProgramLine>}
*/
export function* programLines(peek, page) {
let start = page;
if (peek(start) !== Return) return;
for (let count = 0; count < MostLines; ++count) {
const high = peek(start + 1);
const length = peek(start + 3);
if (high & EndOfProgram || length < LineHeaderBytes || start + length > 0xffff) return;
const end = start + length;
yield { number: (high << 8) | peek(start + 2), start, end };
start = end;
}
}
/**
* Whether BASIC, starting a statement at `address`, has one there: it starts one at the colon that ends the statement a
* procedure or function is called into, as at the end of its DEF, and finds it empty.
*/
export const startsStatement = (peek, address) => peek(address) !== Colon;
/** Whether `address` is in `line`'s text or the return that ends it. */
export const holds = (line, address) => address > line.start && address <= line.end;
/** The line holding `address`, or null when it is outside the program, as it is in a command typed at the prompt. */
export function lineHolding(peek, page, address) {
for (const line of programLines(peek, page)) if (holds(line, address)) return line;
return null;
}
export function findLine(peek, page, number) {
for (const line of programLines(peek, page)) if (line.number === number) return line;
return null;
}
/** The address of a line's first statement, after any spaces. */
export function firstStatement(peek, line) {
let at = line.start + LineHeaderBytes;
while (at < line.end && peek(at) === Space) ++at;
return at;
}
/** The line number following the line number token, its bits rearranged so that no byte of it is a return. */
function lineNumberAfterToken(peek, at) {
const mixed = peek(at);
const low = peek(at + 1) ^ ((mixed << 2) & 0xc0);
const high = peek(at + 2) ^ ((mixed << 4) & 0xc0);
return (high << 8) | low;
}
/**
* A line listed as LIST shows it, in statements, each with the addresses it spans up to the colon or return ending it.
* REM, DATA and a star command take the rest of the line, colons and all.
* @param {(address: number) => number} peek
* @param {ProgramLine} line
* @param {Map<number, string>} keywords
* @returns {{text: string, from: number, to: number}[]}
*/
export function listStatements(peek, line, keywords) {
const statements = [];
let text = "";
let from = line.start + LineHeaderBytes;
let quoted = false;
let restOfLine = false;
for (let at = from; at < line.end; ++at) {
const code = peek(at);
if (!quoted && (code === RemToken || code === DataToken || (code === Star && text.trim() === "")))
restOfLine = true;
if (code === Colon && !quoted && !restOfLine) {
statements.push({ text, from, to: at });
text = "";
from = at + 1;
} else if (quoted || code < FirstToken) {
if (code === Quote) quoted = !quoted;
text += code >= Space && code <= LastPrintable ? String.fromCharCode(code) : ".";
} else Iif (code === LineNumberToken) {
text += lineNumberAfterToken(peek, at + 1);
at += 3;
} else {
text += keywords.get(code) ?? `[&${hexbyte(code)}]`;
}
}
statements.push({ text, from, to: line.end });
return statements;
}
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