45 lines
1.3 KiB
Markdown
45 lines
1.3 KiB
Markdown
## Memory Layout
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On a 8080, there are no segments, the addresses are as displayed.
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On on the 8086, a single segment with a value of 0x0100 is assumed.
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This implies that the first 4k of memory are not used.
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The BDOS may be recompiled for starting at a lower address, in this case,
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the minimum memory requirement may be less than 64k.
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|Start|Size|Function|
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|--|--|--|
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|0xF000|4k|BDOS and hw drivers|
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||var|BDOS data|
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|SP|var|Stack, growing down|
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|||Free memory|
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|0x0100|up to stack|Transient Program Area|
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|0x0080|128 bytes|Command line and disk buffer|
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|0x0000|128 bytes|System data area|
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## Application program interface
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It is using the `call 5` convention with CP/M compatible syscall numbers.
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File I/O is done via FCB blocks.
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## Zero page
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|Offset|Size|Usage|
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|--|--|--|
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|0x00|3 bytes|warm reboot jump instruction|
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|0x03|byte|unused (would be iobyte)|
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|0x04|byte|default drive|
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|0x05|3 bytes|syscall jump instruction|
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|0x08|8 bytes|8080 RST 1 interrupt code|
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|0x10|8 bytes|8080 RST 2 interrupt code|
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|0x18|8 bytes|8080 RST 3 interrupt code|
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|0x20|8 bytes|8080 RST 4 interrupt code|
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|0x28|8 bytes|8080 RST 5 interrupt code|
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|0x30|8 bytes|8080 RST 6 interrupt code|
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|0x38|8 bytes|8080 RST 7 interrupt code / debugger|
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|0x40|varies|area reserved for usage by RDOS|
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|0x80|byte|length of command line|
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|0x81|127 bytes|command line with $length bytes plus a trailing 0x0D|
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