diff --git a/spicode.s65 b/spicode.s65
index 0a9da16..138407b 100644
--- a/spicode.s65
+++ b/spicode.s65
@@ -136,18 +136,6 @@ fmt_digit = $32
     rts
 .endproc
 
-; todo error handling
-.proc hex_char_to_int
-    cmp #'A'
-    bge @char
-    sbc #'0'
-    rts
-@char:
-    sbc #('A' - $A)
-    rts
-.endproc
-
-
 ;********************************************************************************
 ; @function Convert any int into hex
 ; @param ARG2-3: Address of output string
@@ -160,9 +148,9 @@ fmt_digit = $32
 ; @modifies X,Y,A
 ;********************************************************************************
 .proc int_to_hex_str
-    cmp #0
-@loop:
+    bit
     beq @rts  ; check done
+@loop:
     ; load next byte
     pha
         lda ARG4,x
@@ -188,11 +176,106 @@ fmt_digit = $32
         plx
     pla
     dec
+    bne @loop
 @rts:
     rts
 .endproc
 
 
+
+;********************************************************************************
+; @function Convert a hex char into a binary number
+; @details
+;  The char must be one of [0-9,a-f,A-F].
+;  All results are only valid if N == 0.
+; @param A: Char to convert
+; @returns N: 0 == success, 1 == invalid char
+; @returns A: The converted number
+;********************************************************************************
+.proc hex_char_to_int
+    sec
+    sbc #'0'
+    bmi @invalid  ; char was in [0, '0')
+    cmp #10
+    bcc @rts  ; A in [0, 10)
+    ; char higher than '9'
+    sbc #('A' - '0')
+    bmi @invalid  ; char was in ('0', 'A')
+    cmp #7
+    bcc @hex_char  ; A in [0, 6]
+    ; char higher than 'F'
+    sbc #('a' - 'A')
+    bmi @invalid  ; char was in ('F', 'a')
+    cmp #17
+    bcc @hex_char  ; A in [0, 6]
+    ; char was in ('f', $ff]
+@invalid:
+    lda #$ff  ; sets N flag
+    rts
+@hex_char:
+    ; carry is not set
+    adc #10
+@rts:
+    rts
+.endproc
+
+
+;********************************************************************************
+; @function Convert a number encoded as hexadecimal string to a binary number
+; @details
+;  The string must only consist of [0-9,a-f,A-F].
+;  All results are only valid if N == 0.
+; @param ARG0-1: Address of string
+; @param A: Number of chars to convert
+; @param Y: Offset onto string so that first char = (ARG0) + Y
+; @returns N: 0 => success, 1 => invalid string
+; @returns A: 0
+; @returns X: Size of the number in bytes
+; @returns Y: Offset onto string, past the end of the number
+; @modifies X,Y,A
+;********************************************************************************
+.proc hex_to_int
+    bit #%11111111  ; check if accumulator is zero
+    beq @rts
+    ldx #0
+    bit #%00000001  ; check if accumulator is even
+    beq @loop  ; even
+    ; not even
+    stz ARG2
+    pha
+    bra @less_sig_char
+@loop:
+    pha
+        ; more significant char
+        lda (ARG0),y  ; load next char
+        iny
+        jsr hex_char_to_int
+        bmi @invalid
+        rol
+        rol
+        rol
+        rol
+        sta ARG2,x
+@less_sig_char:
+        ; less significant char
+        lda (ARG0),y  ; load next char
+        iny
+        jsr hex_char_to_int
+        bmi @invalid
+        ora ARG2,x
+        sta ARG2,x
+        inx
+    pla
+    dec
+    bne @loop
+@invalid:
+@rts:
+    rts
+
+.endproc
+    
+
+
 TEST_FMT: .asciiz "%x -> %x -> %x:)"
 TEST_OUT: .asciiz "TESTOUT"
 HEX_CHARS_UPPER:    .byte "0123456789ABCDEF"