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Add some documentation

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Robbe Van Herck 2020-01-02 12:08:29 +01:00
parent afe2829849
commit bf236fd40a
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2 changed files with 30 additions and 1 deletions
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bootloader/main.asm
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@ -3,7 +3,7 @@ DISK_ID EQU 0x80
KERNEL_START EQU 0x100000
ELF_START EQU 0x8000
NUM_SECTORS EQU 21
NUM_SECTORS EQU 65
org 0x7C00
bits 16

29
docs/bootloader.md Normal file
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@ -0,0 +1,29 @@
# RoBootloader
## Low Memory
| start | end | use |
|---------|---------|---------------------------|
| 0x00000 | 0x004ff | BIOS stuff, don't touch |
| 0x00500 | 0x06bff | _free space_ |
| 0x06c00 | 0x07bff | **Bootloader stack** |
| 0x07c00 | 0x07dff | Bootloader |
| 0x07e00 | 0x07fff | _free space_ |
| 0x08000 | 0x0ffff | **ELF file** (64 sectors) |
| 0x10000 | 0x7ffff | **kernel space** |
| 0x80000 | 0x9ffff | EDBA, partially usable |
| 0xa0000 | 0xfffff | unusable |
## Drive layout
The first 512 bytes are the bootloader, right after that the 32-bit kernel ELF-file, this kernel can be 32KiB big. In the future, this "kernel" can be replaced by a second stage of the bootloader.
## Known issues
### USB Boot
It doesn't boot the kernel from USB, as BIOS does not recognize USB as valid hard drive for int 13, only hard drives and floppies. Thus it loads sectors 2-65 from the hard drive. If this is written on a hard drive, it could work if DRIVE_ID is set correctly.
### Stack setup
As I don't quite get how segments work in real mode, there are most likely errors in the way I set up my stack. I'm going on a _it works on my qemu_ approach currently.