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arch-handbook/boot: add a space between the next word and the previous stop #320

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arch-handbook/boot: add a space between the next word and the previous stop #320

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arch-handbook/boot: add a space between the next word and the previou…
rilysh Jan 3, 2024
dcd5a28
arch-handbook/boot: add a new line and some refactorization
rilysh Jan 14, 2024
9eb158a
arch-handbook/boot: revert the last change in a few places
rilysh Jan 14, 2024
1d9f3ad
arch-handbook/boot: revert more and properly complete one-sentence-pe…
rilysh Jan 14, 2024
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8 changes: 5 additions & 3 deletions documentation/content/en/books/arch-handbook/boot/_index.adoc
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Expand Up @@ -265,7 +265,7 @@ The next block is responsible for the relocation and subsequent jump to the relo

[.programlisting]
....
movw %sp,%si # Source
movw %sp,%si # Source
movw $start,%di # Destination
movw $0x100,%cx # Word count
rep # Relocate
Expand All @@ -283,7 +283,8 @@ As [.filename]#boot0# is loaded by the BIOS to address `0x7C00`, it copies itsel
The source address, `0x7c00`, is copied to register `%si`.
The destination address, `0x600`, to register `%di`.
The number of words to copy, `256` (the program's size = 512 bytes), is copied to register `%cx`.
Next, the `rep` instruction repeats the instruction that follows, that is, `movsw`, the number of times dictated by the `%cx` register.The `movsw` instruction copies the word pointed to by `%si` to the address pointed to by `%di`.
Next, the `rep` instruction repeats the instruction that follows, that is, `movsw`, the number of times dictated by the `%cx` register.
The `movsw` instruction copies the word pointed to by `%si` to the address pointed to by `%di`.
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this is

This is repeated another 255 times.
On each repetition, both the source and destination registers, `%si` and `%di`, are incremented by one.
Thus, upon completion of the 256-word (512-byte) copy, `%di` has the value `0x600`+`512`= `0x800`, and `%si` has the value `0x7c00`+`512`= `0x7e00`; we have thus completed the code _relocation_.
Expand All @@ -296,7 +297,8 @@ Now, `stosw` is executed 8 times.
This instruction copies a `0` value to the address pointed to by the destination register (`%di`, which is `0x800`), and increments it.
This is repeated another 7 times, so `%di` ends up with value `0x810`.
Effectively, this clears the address range `0x800`-`0x80f`.
This range is used as a (fake) partition table for writing the MBR back to disk.Finally, the sector field for the CHS addressing of this fake partition is given the value 1 and a jump is made to the main function from the relocated code.
This range is used as a (fake) partition table for writing the MBR back to disk.
Finally, the sector field for the CHS addressing of this fake partition is given the value 1 and a jump is made to the main function from the relocated code.
Note that until this jump to the relocated code, any reference to an absolute address was avoided.

The following code block tests whether the drive number provided by the BIOS should be used, or the one stored in [.filename]#boot0#.
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