Chapter 21

Page 474

1. You may not expect what you see when you run this program. Why is this? Even though we’re not using the C runtime environment here, we’re still using the operating system for our I/O. The operating system itself echos the characters as we type them. Recall that the operating system buffers keyboard input until we hit ENTER. Our program then reads the characters from the keyboard buffer (connected through STDIN) one at a time and stores each one in our local variable, aChar. The program then writes the character to the screen using a syscall instruction.

    # echoTyping.s
    # Shows your typing
    # Does not use C libraries
            .intel_syntax noprefix
    # Useful constants
            .equ    STDIN,0
            .equ    STDOUT,1
            .equ    enter,'\n'
      # from asm/unistd_64.h
            .equ    READ,0
            .equ    WRITE,1
            .equ    EXIT,60
    # Stack frame
            .equ    aChar,-16
            .equ    localSize,-16
    # Data
            .section    .rodata
    msg:
            .ascii  "Enter up to 127 characters and press enter:\n"
            .equ    msgsize, .-msg
    # Code
            .text               # text segment
            .globl  myStart
            .type   myStart, @function
    myStart:
            push    rbp         # save frame pointer
            mov     rbp, rsp    # set new frame pointer
            add     rsp, localSize  # for local var.
   
            mov     rdx, msgsize    # prompt user
            lea     rsi, msg[rip]
            mov     rdi, STDOUT
            mov     eax, WRITE
            syscall
               
            lea     rbx, aChar[rbp] # place to store input
    rwLoop:
            mov     rdx, 1      # one character
            mov     rsi, rbx
            mov     rdi, STDIN
            mov     eax, READ
            syscall             # request kernel service
   
            mov     rdx, 1      # one character
            mov     rsi, rbx
            mov     rdi, STDOUT
            mov     eax, WRITE
            syscall             # request kernel service
   
            cmp     byte ptr [rbx], enter   # enter key?
            je      allDone     # yes, all done
            inc     rbx         # no, increment pointer
            jmp     rwLoop      # and continue
    allDone:
            mov     rsp, rbp    # yes, restore stack pointer
            pop     rbp         # and caller frame pointer
            mov     eax, EXIT   # end this process
            syscall