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9E22 String Addition

Submitted by Steve Fewell

Name: SWA = SWA + string expression
Starting Address: &9E22
Entry criteria: SWA contains a string value. The BASIC Text pointer
points to the second string to concatenate to the end of the SWA.
Exit: The SWA contains the concatenated strings.


Frstly, push the existing string value in the SWA to the Stack.
Call &A012 to get the result of expression (as there are no higher priority
operators for strings above '+', then the highest level of the expression handler can
be used [A012].
Y=A [to set flags]
If the second value is not a string then generate a Type Mismatch error.

Store X (type of variable?) on stack.
Add the new string's length to the length of the first string (on Stack). If the
result is > 255 then generate a String Too Long error.
X = resulting length (length of new concatenated string).
Store length of new string on Stack.
Y = Length of secong string.
Move each character of the second string (last character first) to the end of the˙
SWA value, so that the last character of the second string is stored in the SWA
at the new string length position, and the first character of the second string
is stored in the SWA at the position of the first string length + 1.
Now the second string is in the appropriate place, we just need to put the first
string back in to the SWA (starting at position &600), to do this we simply
Pop the SWA String value from the Stack. [Note: This won't overwrite any of the 2nd String, as
only the length of the first value is extracted].

Pop the new string length from the Stack, and store it in &36, so that the correct
new length of the SWA is specified.
Pop X from the stack (to restore the variable type information?).
exit with A = 00 (as result is a String).

Disassembly for the String Addition routine

9E22 Q 032 081 188 20 51 BC JSR &BC51 Push SWA to Stack
9E25   032 018 160 20 12 A0 JSR &A012 Get result of expression
9E28   168 A8 TAY
9E29 f 208 102 D0 66 BNE 102 --> &9E91 [JMP &9092 Type Mismatch error]
9E2B   024 18 CLC
9E2C   218 DA PHX
9E2D   178 004 B2 04 LDA (&04)
9E2F e6 101 054 65 36 ADC &36
9E31   176 221 B0 DD BCS -35 --> &9E10 String Too Long error
9E33   170 AA TAX
9E34 H 072 48 PHA
9E35 6 164 054 A4 36 LDY &36
9E37   185 255 005 B9 FF 05 LDA &05FF,Y
9E3A   157 255 005 9D FF 05 STA &05FF,X
9E3D   202 CA DEX
9E3E   136 88 DEY
9E3F   208 246 D0 F6 BNE -10 --> &9E37
9E41   032 210 188 20 D2 BC JSR &BCD2 Pop String from Stack
9E44 h 104 68 PLA
9E45 6 133 054 85 36 STA &36
9E47   250 FA PLX
9E48   169 000 A9 00 LDA#&00
9E4A   128 003 80 03 BRA 3 --> &9E4F Process any more +'s


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