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Diffstat (limited to 'gcd-amd64.S')
| -rw-r--r-- | gcd-amd64.S | 161 |
1 files changed, 161 insertions, 0 deletions
diff --git a/gcd-amd64.S b/gcd-amd64.S new file mode 100644 index 0000000..16be38f --- /dev/null +++ b/gcd-amd64.S @@ -0,0 +1,161 @@ +# This program is for amd64 arch (64 bits) running Linux, Solaris or FreeBSD. +# Written for GNU Assembler (as) with AT&T syntax +# +# Synopsis: +# +# $ gcc -nostdlib [-D OS=LINUX] gcd-amd64.S -o gcd-amd64-linux +# or +# $ cpp [-D OS=LINUX] gcd-amd64.S | as -o gcd-amd64-linux.o \ +# && ld gcd-amd64-linux.o -o gcd-amd64-linux +# +# +# $ gcc -nostdlib -D OS=SOLARIS gcd-amd64.S -o gcd-amd64-solaris +# or +# $ cpp -D OS=SOLARIS gcd-amd64.S | gas -64 -o gcd-amd64-solaris.o \ +# && gld -m elf_x86_64_sol2 gcd-amd64-solaris.o -o gcd-amd64-solaris +# +# $ clang -nostdlib -D OS=FREEBSD gcd-amd64.S -o gcd-amd64-freebsd +# + +#define LINUX 1 +#define SOLARIS 2 +#define FREEBSD 3 + +#ifndef OS +#define OS LINUX +#endif + + +.data + +# Buffer for output: +buffer: + .space 64 # enough for a 64-bit 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 exit 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, %rsi + +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 %rsi # put remainders going from the end of the buffer + mov %dl, (%rsi) + # now rax is a quotient + and %rax, %rax # is quotient == 0 ? + jnz next_digit # quotient is not 0, go on + + # printing the number: +#if OS == LINUX + mov $1, %rax # syscall `write' +#elif OS == SOLARIS + mov $4, %rax # syscall `write' +#elif OS == FREEBSD + mov $4, %rax # syscall `write' +#else +#error bad OS. +#endif + mov $1, %rdi # write to stdout + mov $buf_end, %rdx + sub %rsi, %rdx # rdx is a number of characters to write (buf_end - rsi) + inc %rdx # + new line + syscall # do syscall (print the number at rsi) + 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. + # Example: ./gcd-amd64-linux 11 22 33 +#if OS == FREEBSD + mov %rdi, %rsp +#endif + pop %rcx # Get the number of command-line options (4) + pop %rsi # Get the pointer to the program name (./gcd-amd64-linux), + dec %rcx # minus program name + jz exit # no arguments are given - exiting + + xor %rax, %rax +gcd_loop: + pop %rsi # get next command line argument + mov %rcx, %r8 # save argument counter + mov %rax, %r9 # save current GCD + call str2uint # convert string at rsi to integer at rbx + mov %r9, %rax # restore current GCD + call gcd2 # gcd of rax and rbx (returned by str2uint) + # now rax is a GCD + mov %r8, %rcx # restore argument counter + loop gcd_loop + + call print # print rax with GCD + +exit: +#if OS == LINUX + mov $60, %rax # exit syscall +#elif OS == SOLARIS + mov $1, %rax # exit syscall +#elif OS == FREEBSD + mov $1, %rax # exit syscall +#else +#error bad OS. +#endif + xor %rdi, %rdi # exit code = 0 + syscall + |