CS Lab machines have met their nemesis

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

diff --git a/src/mul_digits_asm.S b/src/mul_digits_asm.S
new file mode 100644 (file)
index 0000000..36e3be5
--- /dev/null
+++ b/src/mul_digits_asm.S
@@ -0,0 +1,40 @@
+.section .text
+.globl mul_digits
+.type mul_digits, @function
+
+#ifdef __x86_64__
+
+# Multiply an array of 64 bit digits by *one* 64 bit digit, modifies the array in place
+mul_digits:
+       movq %rdx, %rcx # rdx is reserved for mulq, use rcx as counter
+       movq $0, %r12 # Overflow register
+       loop:
+               movq %rsi, %rax # Value to multiply in %rax
+               mulq (%rdi) # Multiply, stored in %rdx:%rax (ie: we get TWO digits)
+               
+               # Add overflow from previous operation
+               add %r12, %rax
+               # Upper digit gets saved as next overflow
+               movq %rdx, %r12
+               
+               # Lower digit goes in current array position
+               movq %rax, (%rdi)
+               
+               dec %rcx # Decrement counter
+               jz end_loop # We are done
+               
+               # Move to next element in the array
+               leaq 8(,%rdi,1), %rdi
+               jmp loop # Repeat
+               
+       end_loop:
+       end:
+               movq %r12, %rax # Return overflow
+               ret # We are done
+
+#else
+
+mul_digits:
+       ret
+
+#endif