dawids/lang-bootstrap
1
0
Fork 0
Code Issues Pull requests Projects Releases Packages Wiki Activity Actions

edit readmes

Browse source
This commit is contained in:
pommicket 2022-02-23 23:50:49 -08:00
parent c75af0c8e5
commit b4e22170b4
8 changed files with 54 additions and 52 deletions
Download patch file Download diff file Expand all files Collapse all files

19
00/README.md
View file

@ -75,7 +75,7 @@ version of ELF)
- `02 00` Object type = executable file (not a dynamic library/etc.)
- `3e 00` Architecture x86-64
- `01 00 00 00` Version 1 of ELF, again
- `78 00 40 00 00 00 00 00` **Entry point of the executable** = 0x400078
- `78 00 40 00 00 00 00 00` Entry point of the executable = 0x400078
- `40 00 00 00 00 00 00 00` Program header table offset in bytes from start of file
- `00 00 00 00 00 00 00 00` Section header table offset (we're not using sections)
- `00 00 00 00` Flags (not important to us)
@ -194,16 +194,13 @@ similar to our first call, with two important differences: first, we specify
`0x241` as the second argument. This tells Linux that we are writing to the
file (`O_WRONLY = 0x01`), that we want to create it if it doesn't exist
(`O_CREAT = 0x40`), and that we want to delete any previous contents it had
(`O_TRUNC = 0x200`). Secondly, we're setting the third argument this time. It
(`O_TRUNC = 0x200`). Also, we're setting the third argument this time. It
specifies the permissions our file is created with (`0o755` means user
read/write/execute, group/other read/execute). This is not very important to
the actual execution of the program, so don't worry if you don't know
about UNIX permissions.
read/write/execute, group/other read/execute).
Note that the output file's descriptor will be 4.
Now we can start reading from the file. We're going to loop back to this part of
the code every time we want to read a new hexadecimal number from the input
file.
Now we can start reading the input file. We're going to loop back to this part of
the code every time we want to read a new digit pair.
- `48 b8 03 00 00 00 00 00 00 00` `mov rax, 3`
- `48 89 c7` `mov rdi, rax`
@ -246,7 +243,7 @@ reasons for this which I won't get into here.
- `8a 03` `mov al, byte [rbx]`
Here we put the ASCII code of the first character read from the file into `rax`.
But now we need to turn the ASCII character code into the actual numerical value
But now we need to turn the ASCII character code into the numerical value
of the hex digit.
- `48 89 c3` `mov rbx, rax`
@ -255,8 +252,8 @@ of the hex digit.
- `0f 8c 0f 00 00 00` `jl 0x400136`
This checks if the character code is greater than the character code for the
digit 9, and jumps to a different part of the code if so. This different part of
the code will handle the case of the hex digits `a` through `f`.
digit 9, and jumps to a different part of the code if so. That part
will handle the case of the hex digits `a` through `f`.
- `48 b8 d0 ff ff ff ff ff ff ff` `mov rax, -48`