add get_ModRM

twasm/asm/main.asm

@@ -320,6 +320,29 @@ get_tte_typed_metadata:
 ; ------------------------------------------------------------------------------
 
 get_direct_addressing_ModRM:
+  mov dl, 11b
+  call get_ModRM
+  ret
+
+; ------------------------------------------------------------------------------
+; get_ModRM
+;
+; description:
+; given 2 register tokens and the mod bits, returns the ModR/M byte
+;
+; parameters:
+; di = token table entry `reg`
+; si = token table entry `R/M`
+; dl = lower 2 bits: mod bits. The rest is ignored
+;
+; returned:
+; al = ModR/M byte; the rest of rax is zeroed
+; ------------------------------------------------------------------------------
+
+get_ModRM:
+  and dl, 11b ; mask for mod bits
+  shl dl, 6
+
   ; di = tte
   call get_reg_bits
   ; al = reg bits
@@ -332,12 +355,12 @@ get_direct_addressing_ModRM:
   call get_reg_bits
   ; al = reg bits
 
-  mov dl, al
+  mov cl, al
 
   xor eax, eax
-  or al, 11b << 6 ; mod bits
-  or al, bl       ; reg bits
-  or al, dl       ; R/M bits
+  or al, dl     ; mod bits
+  or al, bl     ; reg bits
+  or al, cl     ; R/M bits
   and rax, 0xFF ; mask for byte
   ret