diff options
Diffstat (limited to '00')
| -rw-r--r-- | 00/README.md | 19 |
1 files changed, 8 insertions, 11 deletions
diff --git a/00/README.md b/00/README.md index b17060e..3ea31be 100644 --- a/00/README.md +++ b/00/README.md @@ -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` |