Add a math library and use macro

Author: commonkestrel

os/math.asm

@@ -0,0 +1,18 @@
+@include <arithmetic>
+@include <flow>
+@include <util>
+
+/// Arguments: A, B
+/// Return: A
+mul:
+    push B, C
+    mv C, B
+    mv B, A
+    jz C, [.done]
+.loop
+    add A, B
+    dec C
+    jnz C, [.loop]
+.done:
+    pop C, B
+    ret

src/assembler/macros.asm

@@ -277,15 +277,23 @@
         add %reg, 1
     }
     ; 16-bit
-    (%low:reg, %high:reg) {
+    (%high:reg, %low:reg) {
         add %low, 1
         adc %high, 0
     }
 }
 
 /// Decrements the given value
-@macro dec (%reg:reg) {
-    sub %reg, 1
+@macro dec {
+    ; 8-bit
+    (%reg:reg) {
+        sub %reg, 1
+    }
+    ; 16-bit
+    (%high:reg, %low:reg) {
+        sub %low, 1
+        sbb %high, 0
+    }
 }
 
 /// Bitwise inverts the given byte
@@ -310,3 +318,5 @@
 @macro nop () {
     mv A, A ; shortest instruction with no side effects: 3 clock cycles
 }
+
+@macro use(%label:ident|label) {}