Change the adder to a calculator capable of both adding and subtracting and move error messages to their own lines in it

examples/calc.asm

@@ -1,4 +1,4 @@
-        ;Two-digit hexadecimal adder
+        ;Hexadecimal adder and subtractor for two-digit unsigned values
 
         ;Print a prompt
 start:  load    r0, prompt
@@ -13,9 +13,16 @@ start:  load    r0, prompt
         ;Print a newline
         cleq    r0, r0, newln
 
-        ;Print a plus sign
+        ;Read the operation sign
-        load    r0, plus
+        load    r0, ffff
-        store   ffff, r0
+        load    r1, plus
+        breq    r0, r1, storsg
+        load    r1, minus
+        breq    r0, r1, storsg
+        ;Error and halt if the character is not a sign
+        breq    r0, r0, error
+storsg: store   sign, r0
+        ;Print a space
         load    r0, space
         store   ffff, r0
 
@@ -34,19 +41,55 @@ start:  load    r0, prompt
         load    r3, equals
         store   ffff, r3
 
-        ;Add the arguments and load the result to r3
+        ;Check the operation
+        load    r3, sign
+        load    r1, minus
+        breq    r1, r3, brsub
+
+        ;Add and load the result to r3
+        ;Addition
         cleq    r0, r0, adder
+        ;Load the result to r3
+        xor     r3, r3
+        xor     r3, r0
+        ;Skip the subtraction code
+        breq    r0, r0, nosign
+
+        ;Subtract and load the result to r3
+        ;Subtraction
+brsub:  cleq    r0, r0, sub
+        ;Load the result to r3
         xor     r3, r3
         xor     r3, r0
 
-        ;Convert and print the result
+        ;Print a minus sign and unnegate the result if negative
-        ;Overflow
+        ;Check the sign
         xor     r0, r0
+        breq    r0, r1, nosign
+        ;Print a minus sign
+        load    r0, minus
+        store   ffff, r0
+        ;Un-negate the result
+        store   arg1, r3
+        load    r2, one
+        cleq    r0, r0, sub
+        nand    r0, r0
+        ;Reload the result to r3
+        xor     r3, r3
+        xor     r3, r0
+        ;Skip printing the first digit
+        breq    r0, r0, skip
+
+        ;Print the overflow for addition or zero for positive results of
+        ;subtraction
+nosign: xor     r0, r0
         xor     r0, r1
         cleq    r0, r0, n2hex
         store   ffff, r0
+
+        ;Convert and print the result
         ;Restore the result back to r0
-        xor     r0, r0
+skip:   xor     r0, r0
         xor     r0, r3
         ;High nibble
         shr     r0
@@ -68,6 +111,13 @@ start:  load    r0, prompt
         ;Loop
         breq    r0, r0, start
 
+        ;Error
+error:  cleq    r0, r0, newln
+        load    r0, qmark
+        store   ffff, r0
+        cleq    r0, r0, newln
+        halt
+
         ;Read and convert an argument and load it to r0
         ;First digit
 rarg:   load    r0, ffff
@@ -89,10 +139,7 @@ rarg2:  load    r0, ffff
         breq    r3, r1, rarg2
         load    r1, cr
         breq    r3, r1, rarg3
-        load    r0, qmark
+        breq    r0, r0, error
-        store   ffff, r0
-        cleq    r0, r0, newln
-        halt
         ;Combine the digits
 rarg3:  or      r0, r2
         ;Return
@@ -151,11 +198,8 @@ hex2n:  load    r1, tbl01
         breq    r0, r1, vale
         load    r1, tbl2b
         breq    r0, r1, valf
-        ;Print a question mark and halt if the character is not a digit
+        ;Error and halt if the character is not a digit
-        load    r0, qmark
+        breq    r0, r0, error
-        store   ffff, r0
-        cleq    r0, r0, newln
-        halt
         ;Load the value of the hexadecimal digit
 val0:   load    r0, tbl00
         breq    r0, r0, h2nend
@@ -219,6 +263,25 @@ nvrflw: breq    r2, r3, addend
 addend: load    r1, ovrflw
         ret
 
+        ;Subtract r2 from arg1 and store the result in r0 with the sign
+        ;bit stored in r1
+        ;Negate the second argument
+sub:    load    r0, one
+        nand    r2, r2
+        cleq    r0, r0, adder
+        ;Load the first argument to r2
+        load    r2, arg1
+        ;Subtract
+        cleq    r0, r0, adder
+        ;Invert the sign bit
+        xor     r3, r3
+        breq    r3, r1, submin
+        xor     r1, r1
+        breq    r0, r0, subend
+submin: load    r1, one
+        ;Return
+subend: ret
+
         ;Get the hexadecimal digit of a nibble
         ;Extract the low nibble
 n2hex:  load    r1, mask
@@ -296,6 +359,7 @@ n2hend: ret
 space:  data    20
 prompt: data    3e
 plus:   data    2b
+minus:  data    2d
 equals: data    3d
 qmark:  data    3f
 backsp: data     8
@@ -357,6 +421,7 @@ tbl29:  data    65
 tbl2a:  data    f
 tbl2b:  data    66
 
-        ;First argument
+        ;Variables
 ovrflw: data     0
 arg1:   data     0
+sign:   data     0