evaluate constants as long as they are hex in the form 0x

twasm/asm/main.asm

@@ -720,6 +720,82 @@ tokenise:
   .msg_operand db "operand.", 0x0A, 0x00
   .pending_operator dd 0 ; the operator token that is pending processing
 
+; ------------------------------------------------------------------------------
+; evaluate_constant
+;
+; description:
+; takes a constant and returns its hexidecimal representation. Currently the
+; following constants are supported:
+;
+; | type | p. | description  |
+; |------|----|--------------|
+; | 0x00 | 0x | hexidecimal  |
+; | 0xFF |    | unrecognised |
+;
+; where `p.` is the prefix
+;
+; parameters:
+; rdi -> first byte of constant
+; rsi = size of constant in bytes
+;
+; returned:
+; rax = value of the constant in hexidecimal
+; dl = type of constant; the rest of rdx is zeroed
+; ------------------------------------------------------------------------------
+
+evaluate_constant:
+  ; TODO fix this cheap trick xD
+  mov dl, [rdi]
+  cmp dl, '0'
+  jne .unrecognised
+  dec rsi ; one fewer byte left
+  inc rdi ; point to next byte
+
+  mov dl, [rdi]
+  cmp dl, 'x'
+  jne .unrecognised
+  dec rsi ; one fewer byte left
+  inc rdi ; point to next byte
+
+               ; rsi = number of bytes left
+               ; rdi -> current byte of constant
+  xor eax, eax ; rax = value in hex of constant
+
+  .loop:
+    cmp rsi, 0 ; make sure we're in range
+    je .break  ; if not, break
+
+    shl rax, 4 ; make room for next hex digit
+
+    mov dl, [rdi] ; dl = next byte of constant
+
+    sub dl, '0' ; dl = if digit: digit; else :shrug:
+
+    cmp dl, 9     ; if !digit:
+    jg .alpha     ;   letter
+    jmp .continue ; else loop
+
+  .alpha
+    sub dl, 7        ; map [('A'-'0')..('F'-'0')] to [0xA..0xF]
+    cmp dl, 0xF      ; if not in the range [0xA..0xF]
+    jg .unrecognised ; then unrecognised
+
+  .continue
+    and dl, 0x0F ; mask
+    or al, dl    ; and add newest nibble
+
+    dec rsi   ; one fewer byte left
+    inc rdi   ; point to next byte
+    jmp .loop ; and loop
+
+  .break:
+    mov rdx, 0x00 ; hex type
+    ret
+
+  .unrecognised:
+    mov rdx, 0xFF ; unrecognised type
+    ret
+
 ; ------------------------------------------------------------------------------
 ; utilities
 ; ------------------------------------------------------------------------------