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
 ; ------------------------------------------------------------------------------

twasm/asm/tests.asm

@@ -31,6 +31,9 @@ run_tests:
   call clear_test_arena
   call test_get_reg_bits
 
+  call clear_test_arena
+  call test_evaluate_constant
+
   ret
   .msg db "running test suite...", 0x0A, 0x00
 
@@ -300,6 +303,71 @@ test_get_reg_bits:
     ret
   .msg db "test_get_reg_bits...", 0x00
 
+; ------------------------------------------------------------------------------
+; test_evaluate_constant
+;
+; description:
+; tests evaluate_constant described funtionality
+; ------------------------------------------------------------------------------
+
+test_evaluate_constant:
+  mov rsi, .msg
+  call print.test
+
+  ; just numerals
+  mov rdi, .case0 ; addr of constant
+  mov rsi, 8      ; length of constant
+  call evaluate_constant
+  cmp rax, [.case0_solution]
+  jne .fail
+  cmp rdx, 0x00
+  jne .fail
+
+  ; just chars
+  mov rdi, .case1 ; addr of constant
+  mov rsi, 8      ; length of constant
+  call evaluate_constant
+  cmp rax, [.case1_solution]
+  jne .fail
+  cmp rdx, 0x00
+  jne .fail
+
+  ; just chars
+  mov rdi, .case2 ; addr of constant
+  mov rsi, 12     ; length of constant
+  call evaluate_constant
+  cmp rax, [.case2_solution]
+  jne .fail
+  cmp rdx, 0x00
+  jne .fail
+
+  ; PI
+  mov rdi, .case3 ; addr of constant
+  mov rsi, 18     ; length of constant
+  call evaluate_constant
+  cmp rax, [.case3_solution]
+  jne .fail
+  cmp rdx, 0x00
+  jne .fail
+
+  .pass:
+    mov rsi, msg_pass
+    call print
+    ret
+  .fail:
+    mov rsi, msg_fail
+    call print
+    ret
+  .msg db "test_evaluate_constant...", 0x00
+  .case0 db "0x012390"
+  .case0_solution dq 0x012390
+  .case1 db "0xABCDEF"
+  .case1_solution dq 0xABCDEF
+  .case2 db "0x1234567890"
+  .case2_solution dq 0x1234567890
+  .case3 db "0x243F6A8885A308D3"
+  .case3_solution dq 0x243F6A8885A308D3
+
 msg_pass:
   db 0x0A
   times (TEST_LINE_LENGTH + .start - .end) db " ", ; right align