diff --git a/ansi.ts b/ansi.ts
index 8b73b09..2927656 100644
--- a/ansi.ts
+++ b/ansi.ts
@@ -20,7 +20,7 @@ export class AnsiTerminalDisplay implements Display {
 
   constructor(
     private writer: Deno.Writer = Deno.stdout,
-    private reader: Deno.Reader = Deno.stdin,
+    reader: Deno.Reader = Deno.stdin,
   ) {
     if (hasRawMode(reader)) {
       this.readKeyPresses(reader); // purposefully not awaited
diff --git a/base.test.ts b/base.test.ts
index 1f7ab5e..12aecb4 100644
--- a/base.test.ts
+++ b/base.test.ts
@@ -1,4 +1,4 @@
-import { BufferDrawing, CharBuffer } from "./base.ts";
+import { BufferDrawing, BufferLocation, CharBuffer } from "./base.ts";
 import { describe, expect, it } from "./testing.ts";
 
 describe(CharBuffer, () => {
@@ -11,6 +11,26 @@ describe(CharBuffer, () => {
     expect(buffer.get({ x: 0, y: 0 })).toEqual({ ch: " ", fg: 0, bg: 0 });
     expect(buffer.get({ x: 1, y: 1 })).toEqual({ ch: "y", fg: 2, bg: 5 });
   });
+
+  it("keeps track of dirty lines", async () => {
+    const buffer = new CharBuffer({ w: 16, h: 3 });
+    expect(buffer.isDirty({ x: 0, y: 0 })).toBe(false);
+    buffer.set({ x: 0, y: 0 }, { ch: "x", bg: 0, fg: 0 });
+    expect(buffer.isDirty({ x: 0, y: 0 })).toBe(true);
+    expect(buffer.isDirty({ x: 1, y: 0 })).toBe(true);
+    expect(buffer.isDirty({ x: 2, y: 0 })).toBe(false);
+    expect(buffer.isDirty({ x: 0, y: 1 })).toBe(false);
+    let calls: BufferLocation[] = [];
+    await buffer.forEachDirty(async (at) => {
+      calls.push(at);
+    });
+    expect(calls).toEqual([{ x: 0, y: 0 }, { x: 1, y: 0 }]);
+    calls = [];
+    await buffer.forEachDirty(async (at) => {
+      calls.push(at);
+    });
+    expect(calls).toEqual([]);
+  });
 });
 
 describe(BufferDrawing, () => {
diff --git a/base.ts b/base.ts
index 9fe61fe..60eb320 100644
--- a/base.ts
+++ b/base.ts
@@ -25,19 +25,25 @@ export const SPACE: Char = { ch: " ", bg: 0, fg: 0 } as const;
 
 /**
  * Rectangular buffer of displayable characters
+ *
+ * All methods in this class do not properly check for
+ * out-of-bounds coordinates, use BufferDrawing for this
  */
 export class CharBuffer {
   private chars: Array<Char>;
+  private dirty: boolean;
+  private dirty_lines: Uint8Array;
+  private dirty_seg_length: number;
 
   constructor(private size: BufferSize) {
     this.chars = new Array(size.w * size.h).fill(SPACE);
+    this.dirty = false;
+    this.dirty_lines = new Uint8Array(size.h).fill(0x00);
+    this.dirty_seg_length = Math.ceil(size.w / 8);
   }
 
   /**
-   * Get the character buffered at a given at
-   *
-   * This does not properly check for out-of-bounds coordinates,
-   * use BufferDrawing for this
+   * Get the character buffered at a given location
    */
   get(at: BufferLocation): Char {
     const i = at.y * this.size.w + at.x;
@@ -50,17 +56,60 @@ export class CharBuffer {
 
   /**
    * Change the character buffered at a given location
-   *
-   * This does not properly check for out-of-bounds coordinates,
-   * use BufferDrawing for this
    */
   set(at: BufferLocation, char: Char): void {
     const i = at.y * this.size.w + at.x;
     if (i >= 0 && i < this.chars.length) {
+      // TODO only if changed?
       this.chars[i] = char;
+      this.dirty = true;
+      this.dirty_lines[at.y] |= 1 << Math.floor(at.x / this.dirty_seg_length);
     }
   }
 
+  /**
+   * Call a method for each dirty character, then clear the dirty flags
+   */
+  async forEachDirty(
+    op: (at: BufferLocation, char: Char) => Promise<void>,
+  ): Promise<void> {
+    if (this.dirty) {
+      const { w, h } = this.getSize();
+      for (let y = 0; y < h; y++) {
+        const line = this.dirty_lines[y];
+        if (line) {
+          for (let sx = 0; sx < 8; sx++) {
+            if (line & 1 << sx) {
+              const limit = Math.min((sx + 1) * this.dirty_seg_length, w);
+              for (let x = sx * this.dirty_seg_length; x < limit; x++) {
+                await op({ x, y }, this.get({ x, y }));
+              }
+            }
+          }
+        }
+      }
+      this.setDirty(false);
+    }
+  }
+
+  /**
+   * Set the dirty state for all characters
+   */
+  setDirty(dirty: boolean): void {
+    this.dirty = dirty;
+    this.dirty_lines.fill(dirty ? 0xFF : 0x00);
+  }
+
+  /**
+   * Check if a given location is potentially dirty
+   *
+   * This ignores the global dirty flag, only checks dirty line segments
+   */
+  isDirty(at: BufferLocation): boolean {
+    const seg = Math.floor(at.x / this.dirty_seg_length);
+    return (this.dirty_lines[at.y] & (1 << seg)) != 0;
+  }
+
   getSize(): BufferSize {
     return this.size;
   }
diff --git a/ui.ts b/ui.ts
index 7f30f1d..704deff 100644
--- a/ui.ts
+++ b/ui.ts
@@ -80,19 +80,15 @@ export class TextUI {
       return;
     }
 
-    // TODO only dirty chars
-    const { w, h } = this.screen.getSize();
-    for (let y = 0; y < h; y++) {
-      for (let x = 0; x < w; x++) {
-        await this.display.setChar({ x, y }, this.screen.get({ x, y }));
-      }
-    }
+    await this.screen.forEachDirty((at, char) =>
+      this.display.setChar(at, char)
+    );
   }
 
   /**
    * Start the event loop, waiting for input
    */
-  async loop(refresh = 1000): Promise<void> {
+  async loop(refresh = 100): Promise<void> {
     while (!this.quitting) {
       for (const key of await this.display.getKeyStrokes()) {
         if (!this.config.ignoreCtrlC && key == "ctrl+c") {