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Write some documentation on zero page layout

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Nero 2019-10-20 19:57:10 +00:00
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README.md
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@ -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. It is using the `call 5` convention with CP/M compatible syscall numbers.
File I/O is done via FCB blocks. 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|