diff --git a/machine.config.js b/machine.config.js
index d8dac63..4e8395d 100644
--- a/machine.config.js
+++ b/machine.config.js
@@ -1,6 +1,7 @@
 module.exports = {
+  "POINTER_TO_START_OF_DISPLAY_MEM": 32,
+  "POINTER_TO_START_OF_KEYPAD_MEM": 33,
   "INITIAL_IP_ADDRESS":  48,
-  "POINTER_TO_START_OF_DISPLAY_MEM": 0,
 
   // max number of times to step the CPU,
   // to stop endless loops
diff --git a/readme.md b/readme.md
index 2fffa76..a68b67c 100644
--- a/readme.md
+++ b/readme.md
@@ -62,11 +62,11 @@ Assemble and run, with debug output:
 
 ## Memory map / Peripherals
 
-- `00   ` - pointer to screen memory
-- `01   ` - pointer to keypad memory
-- `02-0F` - reserved for future use
-- `10-1F` - display (4x4)
-- `20-2F` - keypad? (details TBD)
+- `00-0F` - display (4x4)
+- `10-1F` - keypad? (details TBD)
+- `20   ` - pointer to display memory
+- `21   ` - pointer to keypad memory
+- `22-2F` - reserved for future use / variable storage
 - `30   ` - initial value for IP
 - `30-FF` - free
 
diff --git a/test-programs/fill-display.asm b/test-programs/fill-display.asm
index d52ae34..3b1d935 100644
--- a/test-programs/fill-display.asm
+++ b/test-programs/fill-display.asm
@@ -1,6 +1,6 @@
 ;; Fill display with $FF
 
-LDA $00
+LDA $00 ; Start of display
 STO $21 ; Pixel index
 
 ; store the $FF that we'll use to fill the screen
diff --git a/test-programs/relocate-display-mem.asm b/test-programs/relocate-display-mem.asm
index f9e5cc4..784066b 100644
--- a/test-programs/relocate-display-mem.asm
+++ b/test-programs/relocate-display-mem.asm
@@ -1,6 +1,6 @@
 ;; test re-locating display memory
 LDA $10
-STO $00 ; change pointer-to-display-memory to $10
+STO $20 ; change pointer-to-display-memory to $10
 STO $10
 STO $11
 STO $12