CS Lab machines have met their nemesis

[ipdf/code.git] / src / sub_digits_asm.S

diff --git a/src/sub_digits_asm.S b/src/sub_digits_asm.S
new file mode 100644 (file)
index 0000000..cd4629a
--- /dev/null
+++ b/src/sub_digits_asm.S
@@ -0,0 +1,38 @@
+.section .text
+.globl sub_digits
+.type sub_digits, @function
+
+#ifdef __x86_64__
+
+# Subtract two arrays of 64 bit digits, with carry, modifying the first argument
+# Address at first argument %rdi is array to add and modify
+# Address at second %rsi will be added (not modified)
+# Third argument is counter of number of digits
+# Result in %rax is the final result in the carry flag
+# Exploits the fact that inc and dec do not affect the carry flag
+sub_digits:
+       subq $0, %rax           # Reset the carry/borrow flag
+       loop:
+               movq (%rsi), %rax # Temporarily store digit from second array
+               sbbq %rax, (%rdi) # Subtract digits in second and first array, store in first
+               dec %rdx # Decrement counter
+               jz end_loop # We are done
+               
+               # Move to next element in the first array
+               leaq 8(,%rdi,1), %rdi
+               # Move to next element in the second array
+               leaq 8(,%rsi,1), %rsi
+               jmp loop # Repeat
+       end_loop:
+               movq $0, %rax
+               jnc end
+               movq $1, %rax
+       end:
+               ret # We are done
+
+#else
+
+sub_digits:
+       ret
+
+#endif