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Update 2024-07-27-sys-write-string.md

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0x3bb 2024-07-27 19:55:07 +00:00
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2024-07-27-sys-write-string.md
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@ -26,7 +26,7 @@ The syntax looks like this:
`0xa` is just the ASCII hex representation of a new line. `0xa` is just the ASCII hex representation of a new line.
```asm ```
; sys_write_string.asm ; sys_write_string.asm
section .data section .data
s1 db "s1", 0x0a, 0; terminated string with NL s1 db "s1", 0x0a, 0; terminated string with NL
@ -38,7 +38,7 @@ Many standard libraries add the `NULL` terminator to initialized strings. This i
Although, since `sys_write` does not expect a terminated string, this should be stripped. By taking the address of `s1` minus the terminated character (1 byte/8 bits). Although, since `sys_write` does not expect a terminated string, this should be stripped. By taking the address of `s1` minus the terminated character (1 byte/8 bits).
```asm ```
s1Len equ $-s1-1; offset of s1 - terminator s1Len equ $-s1-1; offset of s1 - terminator
``` ```
@ -46,7 +46,7 @@ s1Len equ $-s1-1; offset of s1 - terminator
- `.bss` stands for _Block Started by Symbol_, and contains uninitialized variables that assign memory at runtime. For this example, it makes sense to define the value in the `.data` block above instead. - `.bss` stands for _Block Started by Symbol_, and contains uninitialized variables that assign memory at runtime. For this example, it makes sense to define the value in the `.data` block above instead.
```asm ```
section .bss section .bss
``` ```
@ -57,7 +57,7 @@ The final section required is to define the entrypoint of the program
- `.text` refers to the code segment: the program's virtual address space that contains executable instructions. - `.text` refers to the code segment: the program's virtual address space that contains executable instructions.
- `main` is our program's entrypoint. - `main` is our program's entrypoint.
```asm ```
section .text section .text
global main global main
``` ```
@ -66,7 +66,7 @@ global main
If the `global` keyword was omitted, then the linker (`ld`) will not see it, since `main` is scoped to the object file: If the `global` keyword was omitted, then the linker (`ld`) will not see it, since `main` is scoped to the object file:
```asm ```
[0x3bb@heimat 2]$ make [0x3bb@heimat 2]$ make
nasm -f elf64 -g -F dwarf sys_write_string.asm -l sys_write_string.lst nasm -f elf64 -g -F dwarf sys_write_string.asm -l sys_write_string.lst
sys_write_string.asm:9: warning: label alone on a line without a colon might be in error [-w+label-orphan] sys_write_string.asm:9: warning: label alone on a line without a colon might be in error [-w+label-orphan]
@ -89,7 +89,7 @@ With the `global` directive set correctly, the [symbol type identifier](https://
## label ## label
The `main` label serves as an alias to the block of instructions defined below The `main` label serves as an alias to the block of instructions defined below
```asm ```
main: main:
push rbp; push base address of stack frame to restore later push rbp; push base address of stack frame to restore later
mov rbp, rsp; copy address of current stack frame mov rbp, rsp; copy address of current stack frame