Linux elf64 examples not using glibc

User Interface

I would venture to say that 80% of the processing that goes on in modern computing systems does not require user interaction, such as kernel code for Linux, OSX and Windows. For those that do, there are two fundamentals which are interactivity via keyboard (pointing devices) and console. This example and others in my series are oriented around text based console (VT100 emulation) and keyboard.

In and of itself, this example is very simple, but it is an essential building block toward more complex algorithms.

Subrtx.asm

        STDIN    equ        0
       STDOUT    equ        1

     SYS_READ    equ        0
    SYS_WRITE    equ        1

    global  gets, strlen, print, atoq

            section .text

As this is intended exclusively for keyboard, the probability of errors is next to none. I would imagine most often, program will be able to contemplate buffer size to circumvent buffer overrun, but that is not guaranteed due to indirection.

; =============================================================================
; Accept canonical input from operator for a maximum of EDX bytes and replace
; terminating CR with NULL.

;        ENTER: RSI = Pointer to input buffer
;               EDX = Maximum number of characters

;        LEAVE: EAX = Number of characters entered
;               R11 = Modified by syscall, all others preserved.

;        FLAGS:  ZF = Null entry, NZ otherwise.
; _____________________________________________________________________________

     gets:  push    rcx
            push    rdi

            xor     eax, eax            ; RAX = SYS_READ
            mov     edi, eax            ; RDI = STDIN
            syscall

; TODO:    Should probably do some error trapping here, especially for
;            buffer overrun, but I'll see if it becomes an issue over time.

            dec     eax                 ; Bump back to CR
            mov     byte [rsi+rax], 0   ; Replace it with NULL

            pop     rdi
            pop     rcx
            ret

To begin with, this was intended to circumvent the need to either code or manually calculate a strings length for write(2). Then I decided to incorporate a delimiter, now it can be used to scan for any character (0 - FF). This opens the possibility for word wrapping text for example, so the label strlen is a bit of a misnomer as one would generally think the result is going to be the number of visible character.

; =============================================================================
; Determine length, including terminating character EOS. Result may include
; VT100 escape sequences.

;        ENTER: RDI = Pointer to ASCII string.
;               RCX   Bits 31 - 08 = Max chars to scan (1 - 1.67e7)
;                           07 - 00 = Terminating character (0 - FF)

;        LEAVE: RAX = Pointer to next string (optional).

;        FLAGS:  ZF = Terminating character found, NZ otherwise (overrun).
; _____________________________________________________________________________

   strlen:  push    rcx                 ; Preserve registers used by proc so
            push    rdi                 ; it's non-destructive except for RAX.

            mov      al, cl             ; Byte to scan for in AL.
            shr     ecx, 8              ; Shift max count into bits 23 - 00

; NOTE: Probably should check direction flag here, but I always set and
;       reset DF in the process that is using it.

            repnz   scasb               ; Scan for AL or until ECX = 0
            mov     rax, rdi            ; Return pointer to EOS + 1

            pop     rdi                 ; Original pointer for proglogue
            jz      $ + 5               ; ZF indicates EOS was found
            mov     rax, rdi            ; RAX = RDI, NULL string
            pop     rcx

            ret

The intent to this procedure is to simplify loop design in the calling procedure.

; =============================================================================
; Display an ASCIIZ string on console that may have embedded VT100 sequences.

;        ENTER: RDI = Points to string

;        LEAVE: RAX = Number of characters displayed, including EOS
;                   = Error code if SF
;               RDI = Points to byte after EOS.
;               R11 = Modified by syscall all others preserved

;        FLAGS:  ZF = Terminating NULL was not found. NZ otherwise
;                SF = RAX is negated syscall error code.
;______________________________________________________________________________

    print:  push    rsi
            push    rdx
            push    rcx

            mov     ecx, -1 << 8        ; Scan for NULL
            call    strlen
            push    rax                 ; Preserve point to next string
            sub     rax, rdi            ; EAX = End pntr - Start pntr
            jz      .done

     ; size_t = write (int STDOUT, char *, size_t length)

            mov     edx, eax            ; RDX = length
            mov     rsi, rdi            ; RSI = Pointer
            mov     eax, SYS_WRITE
            mov     edi, eax            ; RDI = STDOUT
            syscall
            or      rax, rax            ; Sets SF if syscall error
            
; NOTE:    This procedure is intended for console, but in the event STDOUT is
;        redirected by some means, EAX may return error code from syscall.

    .done:  pop     rdi                 ; Retrieve pointer to next string.
            pop     rcx
            pop     rdx
            pop     rsi

            ret

Finally an example of how these functions can be used.

Generic.asm

global  _start

    extern  print, gets, atoq

    SYS_EXIT  equ   60
         ESC  equ   27

       BSize  equ   96

            section .rodata
   Prompt:  db  ESC, '[2J'      ; VT100 clear screen
            db  ESC, '[4;11H'   ;   "   Position cursor to line 4 column 11
            db  'ASCII -> INT64 (binary, octal, hexidecimal, decimal), '
            db  'Packed & Unpacked BCD and floating point conversions'
            db  10, 10, 0, 9, 9, 9, '=> ', 0
            db  10, 9, 'Bye'
            db  ESC, '[0m'      ; VT100 Reset console
            db  10, 10, 0

            section .text
   _start:  pop    rdi
            mov    rsi, rsp
            and    rsp, byte 0xf0       ; Align stack on 16 byte boundary.

            call   main
            mov    rdi, rax             ; Copy return code into ARG0

            mov    eax, SYS_EXIT
            syscall

; int main ( int argc, char *args[] )

     main:  enter   BSize, 0            ; Input buffer on stack
            mov     edi, Prompt
            call    print
            lea     rsi, [rbp-BSize]    ; Establish pointer to input buffer
            mov     edx, BSize          ; Max size for read(2)

    .Next:  push    rdi                 ; Save pointer to "=> "
            call    print
            call    gets
            jz      .done

            call    atoq                ; Convert string pointed to by RSI 

            pop     rdi                 ; Restore pointer to prompt
            jmp     .Next

    .done:  call    print               ; RDI already points to "Bye"
            xor     eax, eax
            leave
            ret

Makefile

OBJECTS = Subrtx.o Generic.o

Generic : $(OBJECTS)
    ld -oGeneric $(OBJECTS)
    readelf -WS Generic
    
Generic.o : Generic.asm
     nasm -g -felf64 Generic.asm
     
Subrtx.o : Subrtx.asm
    nasm -g -felf64 Subrtx.asm

clean:
    rm -f $(OBJECTS) Generic