diff --git a/README.md b/README.md
index 6bd4e6a..0235527 100644
--- a/README.md
+++ b/README.md
@@ -23,3 +23,22 @@ the minimum memory requirement may be less than 64k.
 It is using the `call 5` convention with CP/M compatible syscall numbers.
 
 File I/O is done via FCB blocks.
+
+## Zero page
+
+|Offset|Size|Usage|
+|--|--|--|
+|0x00|3 bytes|warm reboot jump instruction|
+|0x03|byte|unused (would be iobyte)|
+|0x04|byte|default drive|
+|0x05|3 bytes|syscall jump instruction|
+|0x08|8 bytes|8080 RST 1 interrupt code|
+|0x10|8 bytes|8080 RST 2 interrupt code|
+|0x18|8 bytes|8080 RST 3 interrupt code|
+|0x20|8 bytes|8080 RST 4 interrupt code|
+|0x28|8 bytes|8080 RST 5 interrupt code|
+|0x30|8 bytes|8080 RST 6 interrupt code|
+|0x38|8 bytes|8080 RST 7 interrupt code / debugger|
+|0x40|varies|area reserved for usage by RDOS|
+|0x80|byte|length of command line|
+|0x81|127 bytes|command line with $length bytes plus a trailing 0x0D|