diff --git a/src/ide/views/editors.ts b/src/ide/views/editors.ts
index deb682c2..ca4c5e08 100644
--- a/src/ide/views/editors.ts
+++ b/src/ide/views/editors.ts
@@ -25,6 +25,7 @@ import { disassemblyTheme } from "../../themes/disassemblyTheme";
 import { editorTheme } from "../../themes/editorTheme";
 import { mbo } from "../../themes/mbo";
 import { loadSettings, registerEditor, settingsExtensions } from "../settings";
+import { asmSpacesKeymap } from "./tabs";
 import { clearBreakpoint, current_project, lastDebugState, openHeaderFile, platform, qs, runToPC } from "../ui";
 import { createAssetHeaderPlugin } from "./assetdecorations";
 import { createIncludeLinkPlugin } from "./includedecorations";
@@ -230,6 +231,10 @@ export class SourceEditor implements ProjectView {
           return 0;
         }) : [],
 
+        // Asm files: tab/backspace work on inferred tab stops when
+        // "insert spaces when pressing tab" is enabled. Placed before the
+        // settings keymap so it takes precedence over indentMore/insertTab.
+        ...(isAsm ? [keymap.of(asmSpacesKeymap(() => loadSettings().tabsToSpaces))] : []),
         // Keybindings from settings must appear before default keymap.
         ...settingsExtensions(loadSettings()),
         // https://codemirror.net/docs/ref/#commands.defaultKeymap includes
diff --git a/src/ide/views/tabs.ts b/src/ide/views/tabs.ts
index c173e924..58fd5750 100644
--- a/src/ide/views/tabs.ts
+++ b/src/ide/views/tabs.ts
@@ -1,5 +1,6 @@
 import { indentLess, indentMore, insertTab } from "@codemirror/commands";
-import { KeyBinding } from "@codemirror/view";
+import { EditorSelection, EditorState } from "@codemirror/state";
+import { EditorView, KeyBinding } from "@codemirror/view";
 
 export const smartIndentKeymap: KeyBinding[] = [
   { key: "Tab", run: indentMore },
@@ -10,3 +11,216 @@ export const insertTabKeymap: KeyBinding[] = [
   { key: "Tab", run: insertTab },
   { key: "Shift-Tab", run: indentLess },
 ];
+
+// Number of lines scanned in each direction when inferring tab stops.
+const ASM_TABSTOP_SCAN_LINES = 50;
+// Existing tab characters are assumed to advance to the next multiple of this column.
+const TAB_EXPAND_WIDTH = 8;
+// Print tab stop inference results to the browser console (for debugging).
+const ASM_TAB_DEBUG = true;
+
+function dbgTab(msg: string) {
+  if (ASM_TAB_DEBUG) console.log("[asm-tab] " + msg);
+}
+
+// Compute the visual column of character position 'index' within a line,
+// expanding tab characters to the next multiple of TAB_EXPAND_WIDTH.
+function visualCol(text: string, index: number): number {
+  let col = 0;
+  for (let i = 0; i < index; i++) {
+    col = text.charCodeAt(i) == 9 ? (Math.floor(col / TAB_EXPAND_WIDTH) + 1) * TAB_EXPAND_WIDTH : col + 1;
+  }
+  return col;
+}
+
+// Add a candidate stop column to the frequency tally.
+function addStop(col: number, freq: Map<number, number>) {
+  if (col > 0) freq.set(col, (freq.get(col) || 0) + 1);
+}
+
+// Collect candidate tab stops from one asm line: the opcode start, operand
+// start, and comment start columns (in visual columns, tabs expanded to 8).
+// Lines starting at column 0 are assumed to have a label, so the opcode is the
+// second word on those lines.
+function collectStopsFromLine(text: string, freq: Map<number, number>) {
+  // Visual columns of word starts, stopping at the comment.
+  const words: number[] = [];
+  const re = /\S+/g;
+  let m;
+  while ((m = re.exec(text)) !== null) {
+    if (text[m.index] == ';') break;
+    words.push(visualCol(text, m.index));
+  }
+  // Comment start column (';' preceded by whitespace or line start).
+  const cm = /(^|\s);/.exec(text);
+  if (cm) addStop(visualCol(text, cm.index + cm[1].length), freq);
+  // Skip the label if the line starts in column 0.
+  const fields = words.length && words[0] == 0 ? words.slice(1) : words;
+  if (fields.length >= 1) addStop(fields[0], freq); // opcode column
+  if (fields.length >= 2) addStop(fields[1], freq); // operand column
+}
+
+// Minimum number of non-empty lines scanned after the cursor before we also
+// merge in lines from before it (e.g. when the cursor is near the end of file).
+const ASM_TABSTOP_MIN_LINES = 3;
+
+// Infer tab stops for asm files: scan the lines after the current line first;
+// if there aren't many of them (or they yield no stops), also merge in the lines
+// before it. Returns a frequency tally of stop columns (in visual columns,
+// with tabs expanded to 8).
+export function inferAsmTabStops(state: EditorState, pos: number): Map<number, number> {
+  const cur = state.doc.lineAt(pos);
+  const total = state.doc.lines;
+  const scan = (from: number, to: number, step: number, freq: Map<number, number>): number => {
+    let lines = 0;
+    for (let n = from; n != to && lines < ASM_TABSTOP_SCAN_LINES; n += step) {
+      const line = state.doc.line(n);
+      if (line.text.trim()) {
+        collectStopsFromLine(line.text, freq);
+        lines++;
+      }
+    }
+    return lines;
+  };
+  const freq: Map<number, number> = new Map();
+  // Prefer lines after the current line, but merge in lines before it when
+  // there are too few lines below (cursor near end of file).
+  const after = scan(cur.number + 1, Math.min(total, cur.number + ASM_TABSTOP_SCAN_LINES) + 1, 1, freq);
+  if (after < ASM_TABSTOP_MIN_LINES) {
+    scan(cur.number - 1, Math.max(1, cur.number - ASM_TABSTOP_SCAN_LINES) - 1, -1, freq);
+  }
+  return freq;
+}
+
+// Most frequent stop greater than col (ties: smallest column),
+// or the next grid stop as a fallback.
+function nextStop(freq: Map<number, number>, col: number, grid: number): number {
+  let best = -1, bestFreq = 0;
+  for (const [s, f] of freq) {
+    if (s > col && (f > bestFreq || (f == bestFreq && s < best))) {
+      best = s; bestFreq = f;
+    }
+  }
+  if (best >= 0) return best;
+  return (Math.floor(col / grid) + 1) * grid;
+}
+
+// Most frequent stop less than col (ties: largest column),
+// or the previous grid stop as a fallback.
+function prevStop(freq: Map<number, number>, col: number, grid: number): number {
+  let best = -1, bestFreq = 0;
+  for (const [s, f] of freq) {
+    if (s < col && (f > bestFreq || (f == bestFreq && s > best))) {
+      best = s; bestFreq = f;
+    }
+  }
+  if (best >= 0) return best;
+  return Math.max(0, Math.min(Math.floor((col - 1) / grid) * grid, col - 1));
+}
+
+// True if the document region near pos contains literal tab characters;
+// in that case tab stops fall back to a hard grid of 8 columns.
+function docHasTabs(state: EditorState, pos: number): boolean {
+  const cur = state.doc.lineAt(pos);
+  const fromLine = Math.max(1, cur.number - ASM_TABSTOP_SCAN_LINES);
+  const toLine = Math.min(state.doc.lines, cur.number + ASM_TABSTOP_SCAN_LINES);
+  for (let n = fromLine; n <= toLine; n++) {
+    if (state.doc.line(n).text.includes('\t')) return true;
+  }
+  return false;
+}
+
+// Compute the insertion needed for pressing Tab at document position 'head':
+// spaces from the cursor up to the next inferred tab stop.
+function fmtStops(freq: Map<number, number>): string {
+  return "[" + [...freq.entries()].sort((a, b) => a[0] - b[0]).map(([s, f]) => `${s}:${f}`).join(" ") + "]";
+}
+
+export function asmTabChange(state: EditorState, head: number): { from: number, insert: string } | null {
+  const line = state.doc.lineAt(head);
+  const col = visualCol(line.text, head - line.from);
+  const hasTabs = docHasTabs(state, head);
+  const grid = hasTabs ? TAB_EXPAND_WIDTH : state.tabSize;
+  const stops = inferAsmTabStops(state, head);
+  const target = nextStop(stops, col, grid);
+  dbgTab(`Tab @ line ${line.number} col ${col}: stops(col:freq)=${fmtStops(stops)} ${hasTabs ? "grid=8 (file has tabs)" : `grid=${state.tabSize}`} -> target=${target}`
+    + ` | line=${JSON.stringify(line.text.slice(0, col))}+CURSOR`);
+  if (target <= col) return null;
+  return { from: head, insert: " ".repeat(target - col) };
+}
+
+// Compute the deletion range for Backspace at document position 'head'
+// (spaces removed back to the previous inferred tab stop, or a single tab char),
+// or null if default backspace behavior should apply.
+export function asmBackspaceRange(state: EditorState, head: number): { from: number, to: number } | null {
+  const line = state.doc.lineAt(head);
+  const index = head - line.from;
+  if (index == 0) {
+    dbgTab(`Backspace @ line ${line.number} col 0: start of line, using default`);
+    return null;
+  }
+  // If the character before the cursor is a tab, just delete it.
+  if (line.text[index - 1] == '\t') {
+    dbgTab(`Backspace @ line ${line.number} col ${visualCol(line.text, index)}: deleting one tab char`);
+    return { from: head - 1, to: head };
+  }
+  // Only handle cursors inside a pure-whitespace indent region.
+  if (!/^[ \t]+$/.test(line.text.slice(0, index))) {
+    dbgTab(`Backspace @ line ${line.number} col ${visualCol(line.text, index)}: cursor not in leading whitespace, using default`);
+    return null;
+  }
+  const hasTabs = docHasTabs(state, head);
+  const grid = hasTabs ? TAB_EXPAND_WIDTH : state.tabSize;
+  const col = visualCol(line.text, index);
+  const stops = inferAsmTabStops(state, head);
+  const target = prevStop(stops, col, grid);
+  if (target >= col) {
+    dbgTab(`Backspace @ line ${line.number} col ${col}: no stop before cursor, using default`);
+    return null;
+  }
+  // Convert target visual column back to a character index (only spaces ahead of us).
+  const from = head - (col - target);
+  dbgTab(`Backspace @ line ${line.number} col ${col}: stops(col:freq)=${fmtStops(stops)} ${hasTabs ? "grid=8 (file has tabs)" : `grid=${state.tabSize}`} -> deleting cols ${target}..${col}`
+    + ` | line=${JSON.stringify(line.text.slice(0, index))}+CURSOR`);
+  return { from, to: head };
+}
+
+// Insert spaces from the cursor up to the next inferred tab stop.
+function asmInsertSpaces(view: EditorView): boolean {
+  const state = view.state;
+  if (state.selection.ranges.some(r => !r.empty)) return false; // let indentMore handle selections
+  const tr = state.changeByRange(range => {
+    const change = asmTabChange(state, range.head);
+    if (!change) return { range };
+    return {
+      changes: change,
+      range: EditorSelection.cursor(range.head + change.insert.length),
+    };
+  });
+  view.dispatch(tr);
+  return true;
+}
+
+// Delete spaces from the cursor back to the previous inferred tab stop,
+// but only when the cursor sits in leading whitespace.
+function asmDeleteSpacesBackward(view: EditorView): boolean {
+  const state = view.state;
+  const range = state.selection.main;
+  if (!range.empty) return false;
+  const del = asmBackspaceRange(state, range.head);
+  if (!del) return false;
+  view.dispatch(state.update({
+    changes: del,
+    selection: EditorSelection.cursor(del.from),
+  }));
+  return true;
+}
+
+// Keymap for asm editors when "insert spaces when pressing tab" is enabled.
+// The callback lets the caller gate the behavior on current settings.
+export function asmSpacesKeymap(tabsToSpaces: () => boolean): KeyBinding[] {
+  return [
+    { key: "Tab", run: v => tabsToSpaces() && asmInsertSpaces(v) },
+    { key: "Backspace", run: v => tabsToSpaces() && asmDeleteSpacesBackward(v) },
+  ];
+}