1 files changed, 83 insertions, 0 deletions
diff --git a/src/lib/int2str.s b/src/lib/int2str.s
new file mode 100644
index 0000000..82a2aaa
--- /dev/null
+++ b/src/lib/int2str.s
@@ -0,0 +1,83 @@
+# --------------------------------------------
+# FUNCTION: int2str
+# PURPOSE : Convert a 64 bit integer into ascii
+#           into ASCII encoding
+# INPUTS  : rdi = 64 bit integer
+#           rsi = address of buffer
+# OUTPUTS : rax = address of string (points into the provided buffer)
+#           rdx = length of string
+# CLOBBERS: 
+# --------------------------------------------
+.section .text
+.globl int2str
+.type int2str, @function
+int2str:
+    pushq %rbx                      # save callee-saved register
+
+    movq %rdi, %rax                 # integer to convert
+    movq %rsi, %rbx                 # buffer base
+    movq %rsi, %r9                  # save original buffer 
+    leaq 19(%rbx), %r8              # pointer to last char position
+    movq $10, %rcx                  # divisor
+    xorq %r10, %r10                 # sign flag (0 = positive)
+
+    # handle sign
+    testq %rax, %rax
+    jns .convert
+    negq %rax                       # if the number negative we negate it and
+    movq $1, %r10                   # set sign flag
+
+.convert:
+    testq %rax, %rax
+    jnz .loop                       # check if the number is 0
+
+    # if the number is 0, we just set the char to '0', and length to 0.
+    movb $'0', (%r8)
+    movq %r8, %rax
+    movq $1, %rdx
+    jmp .shift_left                 # shift the result leftmust
+
+.loop:
+    xorq %rdx, %rdx                 # clear rdx before division
+    divq %rcx                       # divide rdx:rax by 10
+    addb $'0', %dl                  # add ASCII base
+    movb %dl, (%r8)                 # move digit into buffer
+    decq %r8                        # decrement pointer
+    testq %rax, %rax                # while quotient is not 0, loop
+    jnz .loop
+
+    incq %r8                        # move back to first digit
+
+    # add '-' if negative
+    testq %r10, %r10
+    jns .calc_len
+    decq %r8
+    movb $'-', (%r8)
+
+.calc_len:                          # count length of the string
+    movq %r8, %rax                  # string start
+    leaq 20(%r9), %rdx              # end of buffer
+    subq %rax, %rdx                 # string length
+
+.shift_left:
+    # shift string to leftmost in buffer if not already there
+    cmpq %rax, %r9
+    je .done                        # already leftmost
+    movq %rdx, %rcx                 # rcx = length of string
+    movq %rax, %rsi                 # rsi = source :: start of string
+    movq %r9, %rdi                  # rdi = destination :: original buff start
+    rep movsb                       # copy string left
+        # Found this online. It's a way to copy down values in memoery.
+        # movbs = Move String Byte..
+        #   It copies 1 byte from memoery to memoery and
+        #   incrementing destination and incrementing source.
+        #       Source  ::  %rsi
+        #       Dest    ::  %rdi
+        # rep = repeat wow.
+        #   It's a prefix that repeats an isntructution rcx times
+        # https://stackoverflow.com/questions/27804852/assembly-rep-movs-mechanism
+    movq %r9, %rax                  # return pointer to start
+
+.done:
+    popq %rbx
+    ret