GNU assembler for x86 (32 bits) Linux

1 files changed, 138 insertions, 0 deletions

diff --git a/assembler/gcd-x86-linux.s b/assembler/gcd-x86-linux.s
new file mode 100644
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--- /dev/null
+++ b/assembler/gcd-x86-linux.s
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+# This program is for Linux on Intel x86 arch (32 bits).
+# Written for GNU Assembler (as) with AT&T syntax
+
+# To make an executable binary:
+# gcc -nostdlib gcd-x86-linux.s -o gcd-x86-linux
+# or
+# as gcd-x86-linux.s -o gcd-x86-linux.o && \
+#    ld gcd-x86-linux.o -o gcd-x86-linux
+
+# On 64 bits system:
+# gcc -m32 -nostdlib gcd-x86-linux.s -o gcd-x86-linux
+# or
+# as --32 gcd-x86-linux.s -o gcd-x86-linux.o && \
+#    ld -melf_i386 gcd-x86-linux.o -o gcd-x86-linux
+#
+# To run:
+# ./gcd-x86-linux 11 22 33 121 792
+# (output should be 11)
+
+
+.data
+
+# Buffer for output:
+buffer:
+    .space 32 # enough for 32 bits integer
+buf_end:
+    .byte  10 # new line
+
+
+.text
+.globl _start
+
+
+#  GCD of two numbers.
+#    input:  eax, ebx - two numbers
+#    output: eax - GCD
+#    uses:   eax, ebx, edx
+gcd2:
+    and %ebx, %ebx # is %ebx == 0 ?
+    jz  gcd2_exit  # %ebx == 0, go to exit and return %eax (GCD)
+    xor %edx, %edx # set %edx = 0 */
+    div %ebx       # divide: %edx:%eax / %ebx, actually: %eax / %ebx
+    mov %ebx, %eax # drop quotient in %eax and keep previous %ebx in %eax
+    mov %edx, %ebx # put remainder in %ebx
+    jmp gcd2
+gcd2_exit:
+    ret
+
+
+
+#  Print an unsigned integer in eax.
+#    input: eax - unsigned integer to print
+#    uses:  eax, ebx, ecx, edx, edi, buffer
+print:
+    mov $10,  %ecx # set %ecx = 10 (radix)
+    mov $buf_end, %edi
+
+next_digit:
+    xor %edx, %edx # set %edx = 0
+    div %ecx       # divide: %edx:%eax / %ecx, actually: %eax / %ecx
+    # %edx is a remainder (0-9), it fits into %dl
+    add  $48, %dl  # get ASCII code: 0 => 48 = '0', 1 => 49 = '1'
+    dec %edi       # put remainders going from the end of the buffer
+    mov %dl, (%edi)
+    # now eax is a quotient
+    and %eax, %eax # is quotient == 0 ?
+    jnz next_digit # quotient is not 0, go on
+    
+    # printing the number:
+    mov $4,   %eax # syscall `write'
+    mov $1,   %ebx # write to stdout
+    mov %edi, %ecx # first character to write
+    mov $buf_end, %edx
+    sub %edi, %edx # edx is a number of characters to write (buf_end - edi)
+    inc %edx       # + new line
+    int $0x80      # do syscall (print the number)
+
+    ret
+
+
+#  Convert string into unsigned integer
+#    input:  esi - pointer to string
+#    output: ebx - unsigned integer
+#    uses:   eax, ebx, ecx, edi, direction flag
+str2uint:
+    xor %ecx, %ecx # it will be the string length
+    dec %ecx       # ecx = -1 != 0 for repne
+    xor %eax, %eax # search for 0 (%eax = %al = 0)
+    mov %esi, %edi
+    cld            # Search forward (std - backward)
+    repne scasb    # search for 0 (in %al), incrementing edi, decrementing ecx
+    not %ecx       # invert ecx to have the string length
+    dec %ecx       # minus ending zero
+
+    xor %ebx, %ebx
+str2uint_loop:
+    lodsb # going forward from esi
+    # HINT: assert '0' <= al <= '9'
+    lea    (%ebx, %ebx, 4), %ebx # ebx = 4*ebx + ebx = 5*ebx ;-)
+    lea -48(%eax, %ebx, 2), %ebx # ebx = 2*ebx + %eax - 48
+                                 # ebx is multiplied by 10 each iteration,
+                                 # eax-48 will be multiplied at the next iteration ;-)
+    loop str2uint_loop
+    ret
+
+
+
+# Entry point for the program
+_start:
+
+    # Access command line, see:
+    # http://www.cin.ufpe.br/~if817/arquivos/asmtut/index.html
+    # Example: ./gcd-x86-linux 11 22 33
+    pop %ecx # Get the number of command-line options (4)
+    pop %esi # Get the pointer to the program name (./gcd-x86-linux),
+    dec %ecx # minus program name
+    jz exit  # no arguments are given - exiting
+
+    xor %eax, %eax
+gcd_loop:
+    pop %esi  # get next command line argument
+    # Well, we used all registers, now we DO NEED stack :-)
+    push %ecx # save argument counter
+    push %eax # save current GCD
+    call str2uint # convert string at esi to integer at ebx
+    pop %eax  # restore current GCD
+    call gcd2 # gcd of eax and ebx (returned by str2uint)
+    # now eax is a GCD
+    pop %ecx  # restore argument counter
+    loop gcd_loop
+
+    call print # print eax with GCD
+
+exit:
+    mov $1,   %eax  # exit syscall
+    xor %ebx, %ebx  # exit code = 0
+    int $0x80
+