/libasm

Primary LanguageC

libasm

Small project to learn assembly by re-coding a few functions of the libft (first 42 common core project) in assembly, such as strlen etc

todos

  • code all functions
  • check for errors and set errno, using extern ___error or errno_location

Learning ressources

notes here until I get obsidian

  • assembly as about talking to the cpu
    • cpu reads instructions and executes them one by one with the fetch-execute cycle
      • Program Counter ?
        • indicates where next instruction to load is
      • Instruction Decoder
        • cpu loads from memory address that program counter gives, and that's passed to instruction decoder, who handles what to do with it
        • operator: function to deal with (arithmetic, moving data, etc)
        • operands: memory addresse(s) of data involved in function above
      • Data Bus
        • retrieves the values at the memory addresses involved in the operation we're trying to compute
        • connects CPU to memory. It's actually a wire (a physical wire)
      • Registers
        • registers are in-cpu memory
      • Arithmetic and Logic Unit (ALU)

reference

  • argument registers: rdi rsi rdx r10 r8 r9
  • data (size, define data, reserve data)
    • bytes 1 = db = rb
    • bytes 2 = dw = rw
    • bytes 4 = dd = rd
    • bytes 8 = dq = rq
    • bytes 16 = dt = rt
    • bytes 32 = dq = rq
    • bytes 64 = dq = rq
    • bytes 128 = dq = rq
  • htons (host to network short) by hand. means changing the byte order to network byte order, whatever that is
    • print hex value of a number
    • swap bytes: 0x16D9 becomes 0xD916
  • run strace to debug easily
    • strace ./a.out
  • compilation
    • nasm -f elf64 *.s
    • ld *.o
    • strace ./a.out
  • getting nasm defines
    • create c file that imports includes you're interested in
    • define nasm constants in nasm syntax %define socket SYS_socket
    • sys/syscall.h gives you the syscall numbers corresponding to functio names
    • run gcc -E -P constants.c > constants.txt to get the preprocessed output, including the nasm defines so you can copy paste them in the asm file