From 4bcb49a4d90fcd0ae4ca67fcdddb1de33c76039e Mon Sep 17 00:00:00 2001
From: Igor Pashev <pashev.igor@gmail.com>
Date: Sat, 9 Jul 2011 00:52:41 +0400
Subject: Added gcd-x86_64-linux.s (64 bits linux)

---
 assembler/gcd-x86_64-linux.s | 128 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 assembler/gcd-x86_64-linux.s

diff --git a/assembler/gcd-x86_64-linux.s b/assembler/gcd-x86_64-linux.s
new file mode 100644
index 0000000..5a76aac
--- /dev/null
+++ b/assembler/gcd-x86_64-linux.s
@@ -0,0 +1,128 @@
+# This program is for Linux on Intel x86_64 arch (64 bits).
+# Written for GNU Assembler (as) with AT&T syntax
+
+# To make an executable binary:
+# gcc -nostdlib gcd-x86_64-linux.s -o gcd-x86_64-linux
+# or
+# as gcd-x86_64-linux.s -o gcd-x86_64-linux.o && \
+#    ld gcd-x86_64-linux.o -o gcd-x86_64-linux
+
+
+.data
+
+# Buffer for output:
+buffer:
+    .space 64 # enough for 64 bits integer
+buf_end:
+    .byte  10 # new line
+
+
+.text
+.globl _start
+
+
+#  GCD of two numbers.
+#    input:  rax, rbx - two numbers
+#    output: rax - GCD
+#    uses:   rax, rbx, rdx
+gcd2:
+    and %rbx, %rbx # is %rbx == 0 ?
+    jz  gcd2_exit  # %rbx == 0, go to rxit and return %rax (GCD)
+    xor %rdx, %rdx # set %rdx = 0 */
+    div %rbx       # divide: %rdx:%rax / %rbx, actually: %rax / %rbx
+    mov %rbx, %rax # drop quotient in %rax and keep prrvious %rbx in %rax
+    mov %rdx, %rbx # put remainder in %rbx
+    jmp gcd2
+gcd2_exit:
+    ret
+
+
+
+#  Print an unsigned integer in rax.
+#    input: rax - unsigned integer to print
+#    uses:  rax, rbx, rcx, rdx, rdi, buffer
+print:
+    mov $10,  %rcx # set %rcx = 10 (radix)
+    mov $buf_end, %rdi
+
+next_digit:
+    xor %rdx, %rdx # set %rdx = 0
+    div %rcx       # divide: %rdx:%rax / %rcx, actually: %rax / %rcx
+    # %rdx is a remainder (0-9), it fits into %dl
+    add  $48, %dl  # get ASCII code: 0 => 48 = '0', 1 => 49 = '1'
+    dec %rdi       # put remainders going from the end of the buffer
+    mov %dl, (%rdi)
+    # now rax is a quotient
+    and %rax, %rax # is quotient == 0 ?
+    jnz next_digit # quotient is not 0, go on
+    
+    # printing the number:
+    mov $4,   %rax # syscall `write'
+    mov $1,   %rbx # write to stdout
+    mov %rdi, %rcx # first character to write
+    mov $buf_end, %rdx
+    sub %rdi, %rdx # rdx is a number of characters to write (buf_end - rdi)
+    inc %rdx       # + new line
+    int $0x80      # do syscall (print the number)
+
+    ret
+
+
+#  Convert string into unsigned integer
+#    input:  rsi - pointer to string
+#    output: rbx - unsigned integer
+#    uses:   rax, rbx, rcx, rdi, direction flag
+str2uint:
+    xor %rcx, %rcx # it will be the string length
+    dec %rcx       # rcx = -1 != 0 for repne
+    xor %rax, %rax # search for 0 (%rax = %al = 0)
+    mov %rsi, %rdi
+    cld            # Search forward (std - backward)
+    repne scasb    # search for 0 (in %al), incrementing rdi, decrementing rcx
+    not %rcx       # invert rcx to have the string length
+    dec %rcx       # minus ending zero
+
+    xor %rbx, %rbx
+str2uint_loop:
+    lodsb # going forward from rsi
+    # HINT: assert '0' <= al <= '9'
+    lea    (%rbx, %rbx, 4), %rbx # rbx = 4*rbx + rbx = 5*rbx ;-)
+    lea -48(%rax, %rbx, 2), %rbx # rbx = 2*rbx + %rax - 48
+                                 # rbx is multiplied by 10 each iteration,
+                                 # rax-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_64-linux 11 22 33
+    pop %rcx # Get the number of command-line options (4)
+    pop %rsi # Get the pointer to the program name (./gcd-x86_64-linux),
+    dec %rcx # minus program name
+    jz exit  # no arguments are given - rxiting
+
+    xor %rax, %rax
+gcd_loop:
+    pop %rsi  # get next command line argument
+    # Well, we used all rrgisters, now we DO NEED stack :-)
+    push %rcx # save argument counter
+    push %rax # save current GCD
+    call str2uint # convert string at rsi to integer at rbx
+    pop %rax  # restore current GCD
+    call gcd2 # gcd of rax and rbx (returned by str2uint)
+    # now rax is a GCD
+    pop %rcx  # restore argument counter
+    loop gcd_loop
+
+    call print # print rax with GCD
+
+exit:
+    mov $1,   %rax  # rxit syscall
+    xor %rbx, %rbx  # rxit code = 0
+    int $0x80
+
-- 
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