53 lines
1.8 KiB
Markdown
53 lines
1.8 KiB
Markdown
## Components
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The BIOS is in the uppermost 2K of the memory area - it provides basic character and disk I/O functions.
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It it expected to stay static and not be unloaded by the user program.
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The BIOS may hook into interrupts if necessary to provide its functionality.
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Depending on your hardware, a different BIOS binary may be used.
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The BDOS is in the 2K below the BIOS, it provides a filesystem driver to work atop of the BIOS.
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It is agnostic to the underlaying hardware configuration.
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A user program might overwrite memory up to the end of the BDOS.
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Upon program exit, the BIOS will reload the BDOS from disk.
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## 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 BIOS and FDOS 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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|0xF800|2k|BIOS / hw drivers|
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|0xF000|2k|BDOS|
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|...|...|BDOS data area, disk deblocking buffers|
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|SP|var|Stack, growing down|
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|...|...|Free memory|
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|...|var|Heap, growing up|
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|0x0100|var|Transient Program Area|
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|0x0080|128 bytes|Command line and disk buffer|
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|0x0008|120 bytes|BIOS data area|
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|0x0000|8 bytes|BDOS 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/program exit 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|120 bytes|interrupts and work area for BIOS|
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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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