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| -rw-r--r-- | assembler/gcd-x86_64-linux.s | 128 |
1 files changed, 128 insertions, 0 deletions
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 + |