Add third byte to the result in the adder

examples/adder.asm

@@ -3,6 +3,8 @@
         ;Print a prompt
 start:  load    r0, prompt
         store   ffff, r0
+        load    r0, space
+        store   ffff, r0
 
         ;Read and convert the first argument and store it to arg1
         cleq    r0, r0, rarg
@@ -14,6 +16,8 @@ start:  load    r0, prompt
         ;Print a plus sign
         load    r0, plus
         store   ffff, r0
+        load    r0, space
+        store   ffff, r0
 
         ;Read and convert the second argument and load it to r2
         cleq    r0, r0, rarg
@@ -26,16 +30,21 @@ start:  load    r0, prompt
         ;Print a newline
         cleq    r0, r0, newln
 
+        ;Print an equals sign
+        load    r3, equals
+        store   ffff, r3
+
         ;Add the arguments and load the result to r3
         cleq    r0, r0, adder
         xor     r3, r3
         xor     r3, r0
 
-        ;Print an equals sign
-        load    r0, equals
-        store   ffff, r0
-
         ;Convert and print the result
+        ;Overflow
+        xor     r0, r0
+        xor     r0, r1
+        cleq    r0, r0, n2hex
+        store   ffff, r0
         ;Restore the result back to r0
         xor     r0, r0
         xor     r0, r3
@@ -90,10 +99,10 @@ rarg3:  or      r0, r2
         ret
 
         ;Print a newline
-newln:  load    r1, cr
-        store   ffff, r1
-        load    r1, lf
-        store   ffff, r1
+newln:  load    r3, cr
+        store   ffff, r3
+        load    r3, lf
+        store   ffff, r3
         ret
 
         ;Get the value of a hexadecimal digit
@@ -183,21 +192,32 @@ valf:   load    r0, tbl1e
         ;Return
 h2nend: ret
 
-        ;Add r2 to r0
+        ;Add r2 to r0 with the overflow stored in r1
+        ;Reset overflow
+adder:  xor     r3, r3
+        store   ovrflw, r3
         ;Copy the first argument to r1
-adder:  xor     r1, r1
+addlop: xor     r1, r1
         xor     r1, r0
-        ;Calculate the sum and carry
+        ;Calculate the sum and carry and copy the pre-shift carry to r1
         xor     r0, r2
         and     r2, r1
+        xor     r1, r1
+        xor     r1, r2
         shl     r2
+        ;Check for overflow
+        rol     r1
+        breq    r1, r2, nvrflw
+        ;Store overflow
+        load    r1, one
+        store   ovrflw, r1
         ;Check for no carry
-        xor     r3, r3
-        breq    r2, r3, addend
+nvrflw: breq    r2, r3, addend
         ;Loop
-        breq    r0, r0, adder
-        ;Return
-addend: ret
+        breq    r0, r0, addlop
+        ;Load overflow and return
+addend: load    r1, ovrflw
+        ret
 
         ;Get the hexadecimal digit of a nibble
         ;Extract the low nibble
@@ -273,6 +293,7 @@ dgtf:   load    r0, tbl1f
 n2hend: ret
 
         ;Characters
+space:  data    20
 prompt: data    3e
 plus:   data    2b
 equals: data    3d
@@ -284,6 +305,9 @@ lf:     data     a
         ;Mask
 mask:   data     f
 
+        ;One
+one:    data     1
+
         ;Hexadecimal table
         ;Numeric digits
 tbl00:  data     0
@@ -334,4 +358,5 @@ tbl2a:  data    f
 tbl2b:  data    66
 
         ;First argument
+ovrflw: data     0
 arg1:   data     0