Calling printf

Objectives

1. Understand printf
2. Understand how to use printf in assembly code.
3. Understand how to assemble/link assembly code using printf with g++

Notes

• printf is a call from c that prints
  • It was replaced by streams in c++
  • But it exists in many forms throughout programming.
• Using printf will allow us to
  • Produce output from our programs
  • Without using loops, ifs, functions.
• We will learn all of the the above portions, but not right away.
  • So we will use printf for now.
  • But I hope to return to pure assembly before the semester is over.
• printf
  • Takes a format string
  • Then a list of arguments that match the format string.
• The format string
  • Null terminated
  • plain text is fine
  • \n is included, NASM requires `\n`
  • Variables are formatted with %specifier
    • Specifier is a letter, %d for example.
      • %d - decimal
      • %x hex
      • %f float
      • %c char
      • %s array of characters
    • There are more, see the man page.
  • The specifier can involve format information, see the man page
  • Null terminated
• After this printf takes
  • A variable number of arguments
  • One for each %X in the format string.
• An example: See printExamp.cpp.
• Using pritnf with NASM
  • We will be writing the main program for now, so _start needs to go
    • Replace this with main
  • To call printf
    1. save rbp and adjust the stack
    2. place the address of the format string in rdi
    3. place the parameters in rsi, rdx, rcx, and then on the stack.
    4. place a 0 in rax
    5. call printf
    6. restore the rbp
  • Save rbp and adjust the stack
    • Intel assembly has a stack managed with push/pop
    • When we want to save a value across function calls, we place it on the stack.
    • The base pointer and stack pointer are values dealing with the stack.
    • And the base pointer needs to be saved/restored.
    • So

      push rbp
      mov rbp, rsp
      sub rsp, 8

  • Place the address of the format string into rdi

    section .data
            fmtString: db `The numbers are: %d and %d, %s %d %d %d\n`,0
            Message: db "Hello World!",0
    ...
            mov rdi, fmtString 

    • Note the format string is in ` `
    • We could also have put it in quotes, followed by 0x0A
    • Note 0 is the null terminator, NEEDED
    • Note the string I will print is also null terminated, REQUIRED
    • The format string requires 4 parameters.

    • mov rbp, rsp
      
      mov rdi, fmtString 

  • Move the parameters int rsi, rdx, rcx, r8, r9

    •     mov rdi, fmtString
          mov rsi, 1234
          mov rdx, 4321
          mov rcx, Message
          mov r8,  7777
          mov r9,  888

  • Then any other parameters on the stack.
    • I honestly would call printf twice before I started using the stack.
    • First of all, that is way too complex of a print statement.
    • And secondly to use the stack you need to push the arguments in reverse order.

    • move r9, 888
      
      push 99
      push 100 

    • Finally You need to pop these after the call

    • call printf
      
      pop rax
      pop rax

  • set rax to 0
    • This is required by the linux ABI,
    • Just do it.

    •     push 100
      
          mov rax, 0

  • Call printf
  • Remember to pop off any arguments placed on the stack.
  • Restore the stack

    •     mov rax, 0
      
          call printf
      
          pop rax
          pop rax
      
          mov rsp, rbp
          pop rbp

• See message.asm.
• Assembling and Linking your program:
  • We will use nasm to produce an object file from our code
  • We will then link it with gcc.
  • see the Makefile
  • or by hand

    nasm -f elf64 -g message.asm
    cc   message.o   -o message