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AF47 STRING$

Submitted by Steve Fewell

Description:

Call routine &96AF to obtain the result of the expression after the STRING$( keyword and convert the result to Integer
(or issue 'Type mismatch' error if the result was a String value).
Push the Integer result to the BASIC stack. This is the number of times that the String value (the second parameter) is
required to be repeated.

Call routine &8EF1 to check that the next non-space character is a comma ',' (issuing 'Missing ,' error if it isn't).
Call routine &ADAC to get the result of the expression after the comma and then check for a closing bracket character
')', and issue 'Missing )' error if a closing bracket was not the next non-space character.
If the result of the expression is not a String value then issue 'Type mismatch' error.

Retrieve the Integer value from the BASIC Stack.
Now the IWA contains the number of times to copy the SWA String value and the SWA contains the String value
to be duplicated.

If the length of the String value to duplicate (SWA) is zero then exit with A = 0 (as the SWA already contains the
result - a null string!
Set Y to the String length (from location &36).
Set A to the IWA's LSB value (location &2A), as only the first byte is taken into account when obtaining the number
of copies required.

If A (number of copies) is zero then set the SWA length (location &36) to zero and exit with A = 0, as the result of
'zero copies' is a null String value.
Decrement the IWA value (&2A). If the result is now zero (i.e. the IWA value was 1) then exit with A = 0, as the SWA
contains the correct result - a single copy of the String value.

[&AF64] Set X to zero.
Now X points to the start of the SWA String Value (offset from &0600) and Y points to the start of the location within
the SWA to copy the duplication of the String value to (i.e. the first free character after the current SWA value, offset
from &0600).
[&AF66] Copy the next character from the X offset to the Y offset of the SWA
Increment X to point to the next character of the original SWA's value.
Increment Y to point to the next free character in the 'new' SWA value.
If Y has reached zero then we have reached the end of the SWA storage space (more than 255 characters), so issue the 'String
too long' error message.

If X is less than the SWA Length byte (location &36) then jump back to &AF66 to copy the next character of the
original String's value.
Otherwise, we have duplicated another copy of the String value and added it to the end of the original SWA value,
so decrement the IWA value (location &2A), if &2A is now zero then set the SWA length (location &36) to Y and
exit with A = 0; otherwise, jump back to &AF64 to copy the original String value again.


Disassembly for the STRING$ routine

AF47   032 175 150 20 AF 96 JSR &96AF Get expression result & convert it to Integer
AF4A & 032 038 188 20 26 BC JSR &BC26 Push IWA value to the BASIC Stack [pushi]
AF4D   032 241 142 20 F1 8E JSR &8EF1 Check for ',' and issue error if not found
AF50   032 172 173 20 AC AD JSR &ADAC Evaluate expression & check for ')'
AF53   208 239 D0 EF BNE -17 --> &AF44   'Type mismatch' error
AF55   032 230 188 20 E6 BC JSR &BCE6 Retrieve IWA value from the BASIC Stack [popi]
AF58 6 164 054 A4 36 LDY &36
AF5A   240 030 F0 1E BEQ 30 --> &AF7A
AF5C * 165 042 A5 2A LDA &2A
AF5E   240 029 F0 1D BEQ 29 --> &AF7D
AF60 * 198 042 C6 2A DEC &2A
AF62   240 022 F0 16 BEQ 22 --> &AF7A
AF64   162 000 A2 00 LDX#&00
AF66   189 000 006 BD 00 06 LDA &0600,X
AF69   153 000 006 99 00 06 STA &0600,Y
AF6C   232 E8 INX
AF6D   200 C8 INY
AF6E   240 016 F0 10 BEQ 16 --> &AF80
AF70 6 228 054 E4 36 CPX &36
AF72   144 242 90 F2 BCC -14 --> &AF66
AF74 * 198 042 C6 2A DEC &2A
AF76   208 236 D0 EC BNE -20 --> &AF64
AF78 6 132 054 84 36 STY &36
AF7A   169 000 A9 00 LDA#&00
AF7C ` 096 60 RTS
AF7D 6 133 054 85 36 STA &36
AF7F ` 096 60 RTS
AF80 L 076 016 158 4C 10 9E JMP &9E10 String Too Long error

 


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