The BBC and Master Computer Public Domain Library
B75B ON
Submitted by Steve Fewell
Description:
Get the next non-space character pointed to by BASIC Text Pointer A.
-
If the character is #&85 ('ERROR-token') then jump to &B741 to set the BASIC ERROR pointer, as follows:
- * [&B741] Get the next non-space character after the 'ERROR' keyword
- * If the character is #&87 ('OFF-Token') then jump to &B739 to check for the end of statement (&9BA6),
- and issue 'Syntax error' if the end of statement was not found (i.e. the next character wasn't ':', '<cr>'
- (carriage return) or 'ELSE-token'). Call routine &B2A6 to reset BASIC's ON ERROR pointer, so that it points to
- BASIC's default error handling routine (which starts at location &B2AF) and jump to &9005 to continue
- executing the Program.
- * Otherwise, Set Y to the BASIC Text Pointer A offset (location &0A) and decrement Y (so that it points to
- the first non-space character after the ERROR-keyword (the character that we just checked).
- * Call &9BBC to Add the BASIC Text Pointer A offset (&0A) to the Base BASIC Text Pointer A address
- (&0B-&0C), and to reset the Offset (&0A) to 1.
- * Zero BASIC text pointer A Offset (&0A).
- * Set BASIC's ON ERROR handler (stored at locations &16-&17) to point to the address in BASIC text
- pointer A (&0B-&0C) - so that it points to the program code following the ON ERROR keywords.
- * Jump to &8FAE to skip the rest of the current program line (as this is the error code that is only executed
- when an error occurs) and continue executing the program starting at the next program line.
- * If the character is #&87 ('OFF-Token') then jump to &B739 to check for the end of statement (&9BA6),
A offset value (&0A) to point to the start of this expression.
Call routine &926F to evaluate the expression at BASIC Text Pointer A and issue 'Type mismatch' error is a String
expression is found, a Floating-Point result will be converted to Integer.
Now, the IWA contains the Integer result of the expression (or the contents of the Integer variable specified) and X
contains the next character after the expression. This routine also sets BASIC Text Pointer B to the BASIC Text Pointer A
value.
If the next character (X) is #&E5 (GOTO-token) or #&E4 (GOSUB-token) then increment Y (to skip over the keyword).
If the next character (X) is not #&F2 (PROC-token), #&E5 (GOTO-token) or #&E4 (GOSUB-token) then issue an
'ON syntax' error as a valid ON statement (ON ERROR..., ON...GOTO, ON...GOSUB, ON...PROC) was not found.
[&B774] Push X to the 6502 Stack (this is the Keyword which indicates the type of ON-statement that we are processing).
Check whether IWA bytes &2B-&2D are zero, if they are not the the IWA contains a value greater than 255 - which is
not within the permitted ON range (1-127), so jump to &B7D5 to check each character of the current program line
(starting from the current character), looking for an 'ELSE-token' (#&8B). If an 'ELSE-token' is found then [&B817]
update the BASIC text pointer A offset (&0A) to point to the location of the ELSE-token, and call routine &9C29 to
execute the ELSE-statement. If a carriage return character ('<cr>') is found without finding an 'ELSE-token' then
issue 'ON range' error. Please see the note at the bottom for a flaw with this ELSE-statement search.
[&B77D] Decrement the IWA LSB (&2A) value.
If the IWA LSB (&2A) value is now zero then the original IWA value was 1, so jump to &B7B6 to execute the
PROC/GOTO/GOSUB statement that is pointed to by the BASIC Text Pointer A (i.e. the first statement after the GOTO/
GOSUB/PROC keyword).
[B781] If the IWA (&2A) value was less than 0 or greater than 127 (i.e. negative flag set) after the decrement, then
the value is not within the permitted ON range (1-127), so jump to &B7D5 to check each character of the current program
line (starting from the current character), looking for an 'ELSE-token' (#&8B). If an 'ELSE-token' is found then [&B817]
update the BASIC text pointer A offset (&0A) to point to the location of the ELSE-token, and call routine &9C29 to
execute the ELSE-statement. If a carriage return character ('<cr>') is found without finding an 'ELSE-token' then
issue 'ON range' error. Please see the note at the bottom for a flaw with this ELSE-statement search.
[&B783] If the current character (pointed to by PTR A (&0B-&0C) plus offset Y) is either a carriage return
('<cr>'), a colon (':') or an 'ELSE-token' clause, then the end of the ON statement has been found, (or the ELSE-part
of the ON-statement has been found), this means that the GOSUB,GOTO or PROC statement corresponding to the original
value specified in the IWA was not located within the statement, so jump to &B7D5 to check each character of the current
program line (starting from the current character), looking for an 'ELSE-token' (#&8B). If an 'ELSE-token' is found
then [&B817] update the BASIC text pointer A offset (&0A) to point to the location of the ELSE-token, and call
routine &9C29 to execute the ELSE-statement. If a carriage return character ('<cr>') is found without finding an
'ELSE-token' then issue 'ON range' error. Please see the note at the bottom for a flaw with this ELSE-statement search.
[&B791] Skip the current GOTO, GOSUB or PROC statement, and move forward to the next GOTO line,
GOSUB line or PROC call in the ON-statement (that is the next statement after a comma ',' has been located).
This is done as follows:
Increment Y offset (to point to the next character). The accumulator (A), however, still contains the current character.
If the current character (in A) was a double quote ("), then Exclusive-OR the character with the contents of location
&2B (this flag indicates whether the current character is inside a String literal or not).
If the value of the String Literal flag (location &2B) is not zero, then we are currently inside a String Literal,
so ignore the current character and jump back to &B783 to process the next character.
Note: the initial value of &2B is zero (as the IWA now only contains an 8-bit value).
If the current character is a close bracket ')', then decrement location &2C. Location &2C is a flag that
determines whether we have reached the end of a function call such as MID$(a$,3,4).
If the current character is an open bracket '(' then increment location &2C (the bracket counter).
If the current character is not a comma then jump back to &B783 to process the next character, as we haven't found
the next GOTO, GOSUB or PROC call yet.
Now, we have found a comma. However, if location &2C (the bracket counter) contains a positive value, then the
comma forms part of a function call, and is, therefore, not the ON list separator, so jump back to &B783 to process
the next character.
Now, we have found the next list element in the On-Statement, so decrement the IWA value (&2A) and check the value
of the IWA (location &2A). If the IWA has reached zero, then we have found the ON GOSUB, GOTO or PROC statement
for the original value of the IWA, so jump to &B783 to execute the statement.
Otherwise, we haven't found the GOTO, GOSUB or PROC call that corresponds to the original IWA value, so jump back to
&B783 to process the next character.
[&B7B6] Execute the PROC/GOTO/GOSUB statement:
Retrieve the ON statement type (either PROC/GOTO/GOSUB) from the 6502 stack.
If the statement type is 'PROC' then jump to &B803 to execute the PROCedure call.
Store Y back to the BASIC text pointer A offset (to update the offset value). This didn't need to be done for PROC, as
PROC sets the BASIC Text pointer B offset instead.
-
If the statement type is 'GOSUB' then:
- * [&B7CA] Call routine &B82A to Read the Line Number at BASIC Text pointer A and find the Program address
- for the start of that Program line.
- * Set Y to the BASIC Text pointer A offset (&0A), i.e. the character after the Line Number.
- * Call &B81D to skip the rest of the ON-Statement until a ':' or carriage return ('<cr>') character is
- found. This is so that on return from the GOSUB routine, the program will correctly continue to execute at
- the next program statement.
- * Jump to &B6DC to execute the GOSUB statement by checking for end of statement & GOSUBing the Line
- Number specified
- for the start of that Program line.
-
Otherwise, the statement type must be 'GOTO', so:
- * [&B7C1] Call routine &B82A to Read the Line Number at BASIC Text pointer A and find the Program address
- for the start of that Program line.
- * Call routine &9BC6 to set the BASIC Text Pointer A offset (&0A) to 1 and to check for the Escape-Key
- pressed condition - if the Escape key has been pressed, then issue 'Escape' error.
- * Jump to &B723 to Display the Line Number on the screen (if TRACE is currently active), Set the
- BASIC Text Pointer A Offset to 4 (to skip the 3-byte Line Number and 1-byte offset to the next line which
- are located at the start of the Program Line), Set the BASIC Text Pointer A to the address specified in
- locations &3D-&3E and jump to &900B to start executing the statement at the start of the specified
- Program Line.
- * Note: The rest of the ON-Statement does not need to be skipped (using &B81D), as it is in the GOSUB call,
- as we will not be returning from the GOTO call.
- for the start of that Program line.
[&B803] Execute the PROC statement:
Store Y in location &1B (to update BASIC text pointer B to the current character on the line (pointed to by Y)
- this should be the PROC-token).
Get the next non-space character pointed to by the BASIC text pointer B.
If the character is not PROC (note: that each comma-separated statement in an ON...PROC statement must begin with the
PROC-keyword (i.e. ON...PROCxxx,PROCyyy,PROCzzz)) then issue 'ON syntax' error.
Call the routine &B019 to execute the PROC statement.
Set Y to the BASIC text pointer B offset (the value after the PROCedure name)
Call routine &B81D to skip the rest of the program statement (until a ':' or carriage return ('<cr>') character
is found).
Jump to &9002 to execute the next program statement.
Flaws with the ON routine
The ON routine has one flaw, found in routine &B7D5. This routine keeps checking the rest of the Program line
until either an ELSE-token or a carriage-return character is found.
If an ELSE-token is found then the ELSE-statement will be executed, if a carriage return ('<cr>') character is found
'ON range' error will be issued, as an out of range value was found and the ON statement doesn't have an ELSE-Statement
part.
However, the problem is that the 'ON' keyword is a statement, which ends when a carriage return ('<cr>') or ':'
character is found - however BASIC will keep searching for the 'ELSE-statement' past the end of the ON-Statement, if the
ON-statement terminates with a colon.
-
E.g.:
- 5 ON ERROR A = A / B : GOTO 40
- 10 INPUT A
- 20 INPUT B
- 30 ON A GOSUB 100,110,120,130 : IF B > 0 THEN LET A = A / B ELSE LET A = 0
- 40 PRINT A
- 50 END
- 100 LET A = A * 20:RETURN
- 110 LET A = A * 30:RETURN
- 120 LET A = A * 40:RETURN
- 130 LET A = A * 50:RETURN
- 10 INPUT A
then the ELSE-Statement part of the 'IF-Statement' will be executed - even though the ON-Statement terminated (with a ':'
character) before the IF statement started. This is basically due to BASIC not checking for a colon ':' character as well
as a carriage return. Therefore, in the above example, if A = 5 then A will be set to 0 - instead of being divided by B
(as per the code following the ON ERROR statement, as the ON statement should have issued 'ON Range' error).
Disassembly for the ON routine
| B739 | 032 166 155 | 20 A6 9B | JSR &9BA6 Check end of Statement | |
| B73C | 032 166 178 | 20 A6 B2 | JSR &B2A6 Reset 'ON ERROR' pointer to BASIC's default error handling routine | |
| B73F | 128 217 | 80 D9 | BRA -39 --> &B71A Jump to &9005 to continue running the program | |
| B741 | 032 224 142 | 20 E0 8E | JSR &8EE0 Get next non-space character pointed to by Ptr A | |
| B744 | 201 135 | C9 87 | CMP#&87 | |
| B746 | 240 241 | F0 F1 | BEQ -15 --> &B739 Execute ON ERROR OFF statement | |
| B748 | 164 010 | A4 0A | LDY &0A | |
| B74A | 136 | 88 | DEY | |
| B74B | 032 188 155 | 20 BC 9B | JSR &9BBC Update BASIC Text pointer A (Add offset value & then reset offset to 1) | |
| B74E | d | 100 010 | 64 0A | STZ &0A |
| B750 | 165 011 | A5 0B | LDA &0B | |
| B752 | 133 022 | 85 16 | STA &16 | |
| B754 | 165 012 | A5 0C | LDA &0C | |
| B756 | 133 023 | 85 17 | STA &17 | |
| B758 | L | 076 174 143 | 4C AE 8F | JMP &8FAE Skip the rest of the line and process the next program line |
| B75B | 032 224 142 | 20 E0 8E | JSR &8EE0 Get next non-space character pointed to by Ptr A | |
| B75E | 201 133 | C9 85 | CMP#&85 | |
| B760 | 240 223 | F0 DF | BEQ -33 --> &B741 Execute 'ON ERROR' statement | |
| B762 | 198 010 | C6 0A | DEC &0A | |
| B764 | o | 032 111 146 | 20 6F 92 | JSR &926F Evaluate Expression at BASIC Text pointer A & convert the result to integer |
| B767 | 224 242 | E0 F2 | CPX#&F2 | |
| B769 | 240 009 | F0 09 | BEQ 9 --> &B774 | |
| B76B | 200 | C8 | INY | |
| B76C | 224 229 | E0 E5 | CPX#&E5 | |
| B76E | 240 004 | F0 04 | BEQ 4 --> &B774 | |
| B770 | 224 228 | E0 E4 | CPX#&E4 | |
| B772 | v | 208 118 | D0 76 | BNE 118 --> &B7EA 'ON syntax' error |
| B774 | 218 | DA | PHX | |
| B775 | + | 165 043 | A5 2B | LDA &2B |
| B777 | , | 005 044 | 05 2C | ORA &2C |
| B779 | - | 005 045 | 05 2D | ORA &2D |
| B77B | X | 208 088 | D0 58 | BNE 88 --> &B7D5 Check for ELSE-token (if found then execute ELSE statement) otherwise issue 'ON range' error |
| B77D | * | 198 042 | C6 2A | DEC &2A |
| B77F | 5 | 240 053 | F0 35 | BEQ 53 --> &B7B6 Execute the PROC/GOTO/GOSUB statement |
| B781 | 0R | 048 082 | 30 52 | BMI 82 --> &B7D5 Check for ELSE-token (if found then execute ELSE statement) otherwise issue 'ON range' error |
| B783 | 177 011 | B1 0B | LDA (&0B),Y | |
| B785 | 201 013 | C9 0D | CMP#&0D | |
| B787 | L | 240 076 | F0 4C | BEQ 76 --> &B7D5 Check for ELSE-token (if found then execute ELSE statement) otherwise issue 'ON range' error |
| B789 | : | 201 058 | C9 3A | CMP#&3A |
| B78B | H | 240 072 | F0 48 | BEQ 72 --> &B7D5 Check for ELSE-token (if found then execute ELSE statement) otherwise issue 'ON range' error |
| B78D | 201 139 | C9 8B | CMP#&8B | |
| B78F | D | 240 068 | F0 44 | BEQ 68 --> &B7D5 Check for ELSE-token (if found then execute ELSE statement) otherwise issue 'ON range' error |
| B791 | 200 | C8 | INY | |
| B792 | " | 201 034 | C9 22 | CMP#&22 |
| B794 | 208 004 | D0 04 | BNE 4 --> &B79A | |
| B796 | E+ | 069 043 | 45 2B | EOR &2B |
| B798 | + | 133 043 | 85 2B | STA &2B |
| B79A | + | 166 043 | A6 2B | LDX &2B |
| B79C | 208 229 | D0 E5 | BNE -27 --> &B783 | |
| B79E | ) | 201 041 | C9 29 | CMP#&29 |
| B7A0 | 208 002 | D0 02 | BNE 2 --> &B7A4 | |
| B7A2 | , | 198 044 | C6 2C | DEC &2C |
| B7A4 | ( | 201 040 | C9 28 | CMP#&28 |
| B7A6 | 208 002 | D0 02 | BNE 2 --> &B7AA | |
| B7A8 | , | 230 044 | E6 2C | INC &2C |
| B7AA | , | 201 044 | C9 2C | CMP#&2C |
| B7AC | 208 213 | D0 D5 | BNE -43 --> &B783 | |
| B7AE | , | 166 044 | A6 2C | LDX &2C |
| B7B0 | 208 209 | D0 D1 | BNE -47 --> &B783 | |
| B7B2 | * | 198 042 | C6 2A | DEC &2A |
| B7B4 | 208 205 | D0 CD | BNE -51 --> &B783 | |
| B7B6 | h | 104 | 68 | PLA |
| B7B7 | 201 242 | C9 F2 | CMP#&F2 | |
| B7B9 | H | 240 072 | F0 48 | BEQ 72 --> &B803 Execute the PROC statement |
| B7BB | 132 010 | 84 0A | STY &0A | |
| B7BD | 201 228 | C9 E4 | CMP#&E4 | |
| B7BF | 240 009 | F0 09 | BEQ 9 --> &B7CA | |
| B7C1 | * | 032 042 184 | 20 2A B8 | JSR &B82A Get Line Number & find Program Address of the Line Number |
| B7C4 | 032 198 155 | 20 C6 9B | JSR &9BC6 Set PTR A Offset to 1 & Check for Escape error condition | |
| B7C7 | L# | 076 035 183 | 4C 23 B7 | JMP &B723 Jump to Program Line spec ified |
| B7CA | * | 032 042 184 | 20 2A B8 | JSR &B82A Get Line Number & find Program Address of the Line Number |
| B7CD | 164 010 | A4 0A | LDY &0A | |
| B7CF | 032 029 184 | 20 1D B8 | JSR &B81D Skip the rest of the program statement (until a ':' or carriage return ('<cr>') character is found) | |
| B7D2 | L | 076 220 182 | 4C DC B6 | JMP &B6DC Check for end of statement & GOSUB the Line Number specified in the IWA |
| B7D5 | h | 104 | 68 | PLA |
| B7D6 | 177 011 | B1 0B | LDA (&0B),Y | |
| B7D8 | 200 | C8 | INY | |
| B7D9 | 201 139 | C9 8B | CMP#&8B | |
| B7DB | : | 240 058 | F0 3A | BEQ 58 --> &B817 Update PTR A Offset & execute the ELSE-statement |
| B7DD | 201 013 | C9 0D | CMP#&0D | |
| B7DF | 208 245 | D0 F5 | BNE -11 --> &B7D6 | |
| B7E1 | 'ON range' error | |||
| B7EA | 'ON syntax' error | |||
| B7F4 | 'No such line' error | |||
| B803 | 132 027 | 84 1B | STY &1B | |
| B805 | 032 213 142 | 20 D5 8E | JSR &8ED5 Get next non-space character (PTR B) | |
| B808 | 201 242 | C9 F2 | CMP#&F2 | |
| B80A | 208 222 | D0 DE | BNE -34 --> &B7EA 'ON syntax' error | |
| B80C | 032 025 176 | 20 19 B0 | JSR &B019 PROC | |
| B80F | 164 027 | A4 1B | LDY &1B | |
| B811 | 032 029 184 | 20 1D B8 | JSR &B81D Skip the rest of the program statement (until a ':' or carriage return ('<cr>') character is found) | |
| B814 | L | 076 002 144 | 4C 02 90 | JMP &9002 'Syntax error' if not end of statement; otherwise, execute next statement/program line |
| B817 | 132 010 | 84 0A | STY &0A | |
| B819 | L) | 076 041 156 | 4C 29 9C | JMP &9C29 Find and execute the ELSE statement (if one is found) |
| B81C | 200 | C8 | INY | |
| B81D | 177 011 | B1 0B | LDA (&0B),Y | |
| B81F | 201 013 | C9 0D | CMP#&0D | |
| B821 | 240 004 | F0 04 | BEQ 4 --> &B827 | |
| B823 | : | 201 058 | C9 3A | CMP#&3A |
| B825 | 208 245 | D0 F5 | BNE -11 --> &B81C | |
| B827 | 132 010 | 84 0A | STY &0A | |
| B829 | ` | 096 | 60 | RTS |
