44 lines
1.7 KiB
Markdown
44 lines
1.7 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, but different depending on the instruction set.
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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 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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|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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|0x0005|3 bytes |Jump code for BDOS|
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|0x0004|1 bytes |default drive for BDOS|
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|0x0003|1 bytes |may be used as iobyte by BIOS|
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|0x0000|3 bytes |Jump to BIOS warm boot entry point / program exit|
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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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