The BBC and Master Computer Public Domain Library
AEC5 MID$(
Submitted by Steve Fewell
Description:
Call routine &9D3B to evaluate the expression at BASIC text pointer B and issue a 'Type mismatch' error if a String
value was not returned.
If the next non-space character (stored in X) is not a comma then issue 'Missing ,' error; otherwise, increment BASIC text
pointer B offset to skip past the comma value and push the SWA value to the BASIC stack.
Get the Integer result of the expression at BASIC Text pointer B ('Type mismatch' error if value is not numeric or convert
to Integer if Floating-Point value).
Store &2A (the LSB of the Integer value - i.e. the starting point within the SWA value) on the 6502 processor Stack.
Set &2A to value #&FF (i.e. default value for Number of characters is #&FF, or the maximum number of characters
(which will return all characters from the Starting point to the end of SWA is reached)).
Increment BASIC text pointer B offset to skip past the next character (which should be either a comma or closing parenthesis).
If the next character (stored in X) was neither a Comma or closing parenthesis, then issue 'Missing ,' error.
If the next character (stored in X) was a Comma (','), then [&96A7] Extract the Integer result (after the comma) and
check for a closing parenthesis character (')'). If close bracket was not found then issue 'Syntax error'.
Retrieve the SWA String value from the Stack.
Retrieve the Integer Starting Position from the 6502 Stack and store the value in Y.
-
If Y is not zero (i.e. not starting at the beginning of the String Value) then:
- * Subtract the SWA length (&36) + 1 from the Starting position.
- * If the result did not underflow (SWA length is less than the Starting position), so exit with SWA set to a null string
- (length 0) and exit.
- * Decrement Y (The Starting Position value)
- * If the result did not underflow (SWA length is less than the Starting position), so exit with SWA set to a null string
needed because the first SWA character starts at 0, whereas the first String character in BASIC starts at 1.
Store the (Starting Position - 1, i.e. the Y value before the above calculation) in X register and location &2C.
Set Y to 0.
Subtract &2C (Starting Position-1) from the SWA length (&36) to obtain the number of characters from the Starting
position to the end of the SWA value.
If the result is less than &2A (number of characters to take) then set &2A (number of characters to take) to A (the
number of characters in the SWA between the Starting point and the end of the SWA value).
If &2A (Number of characters to take) is zero then set the SWA value to a null string (length 0) and exit.
Otherwise, Keep copying the character at SWA location X (Starting position) to SWA location Y (start of SWA value, initially 0)
and then incrementing X and Y until Y is equal to &2A (number of characters to copy).
Set the SWA length (&36) to Y and exit with A = 0 (to indicate that the result is a String value).
Disassembly for the MID$( routine
| AEBF | L | 076 146 144 | 4C 92 90 | JMP &9092 Type mismatch error |
| AEC2 | L | 076 246 142 | 4C F6 8E | JMP &8EF6 'Missing ,' error |
| AEC5 | ; | 032 059 157 | 20 3B 9D | JSR &9D3B Evaluate expression at BASIC Text pointer B |
| AEC8 | 208 245 | D0 F5 | BNE -11 --> &AEBF [JMP &9092 Type mismatch error] | |
| AECA | , | 224 044 | E0 2C | CPX#&2C |
| AECC | 208 244 | D0 F4 | BNE -12 --> &AEC2 [JMP &8EF6 'Missing ,' error] | |
| AECE | Q | 032 081 188 | 20 51 BC | JSR &BC51 Push SWA to Stack |
| AED1 | 230 027 | E6 1B | INC &1B | |
| AED3 | 032 175 150 | 20 AF 96 | JSR &96AF Get expression result & convert it to Integer | |
| AED6 | * | 165 042 | A5 2A | LDA &2A |
| AED8 | H | 072 | 48 | PHA |
| AED9 | 169 255 | A9 FF | LDA#&FF | |
| AEDB | * | 133 042 | 85 2A | STA &2A |
| AEDD | 230 027 | E6 1B | INC &1B | |
| AEDF | ) | 224 041 | E0 29 | CPX#&29 |
| AEE1 | 240 007 | F0 07 | BEQ 7 --> &AEEA | |
| AEE3 | , | 224 044 | E0 2C | CPX#&2C |
| AEE5 | 208 219 | D0 DB | BNE -37 --> &AEC2 [JMP &8EF6 'Missing ,' error] | |
| AEE7 | 032 167 150 | 20 A7 96 | JSR &96A7 Extract Integer result of expression & check for ')' | |
| AEEA | 032 210 188 | 20 D2 BC | JSR &BCD2 Pop String (SWA) from the stack | |
| AEED | h | 104 | 68 | PLA |
| AEEE | 168 | A8 | TAY | |
| AEEF | 024 | 18 | CLC | |
| AEF0 | 240 006 | F0 06 | BEQ 6 --> &AEF8 | |
| AEF2 | 6 | 229 054 | E5 36 | SBC &36 |
| AEF4 | 176 197 | B0 C5 | BCS -59 --> &AEBB Exit with SWA length (&36) = 0 and A = 0 | |
| AEF6 | 136 | 88 | DEY | |
| AEF7 | 152 | 98 | TYA | |
| AEF8 | , | 133 044 | 85 2C | STA &2C |
| AEFA | 170 | AA | TAX | |
| AEFB | 160 000 | A0 00 | LDY#&00 | |
| AEFD | 6 | 165 054 | A5 36 | LDA &36 |
| AEFF | 8 | 056 | 38 | SEC |
| AF00 | , | 229 044 | E5 2C | SBC &2C |
| AF02 | * | 197 042 | C5 2A | CMP &2A |
| AF04 | 176 002 | B0 02 | BCS 2 --> &AF08 | |
| AF06 | * | 133 042 | 85 2A | STA &2A |
| AF08 | * | 165 042 | A5 2A | LDA &2A |
| AF0A | 240 175 | F0 AF | BEQ -81 --> &AEBB Exit with SWA length (&36) = 0 and A = 0 | |
| AF0C | 189 000 006 | BD 00 06 | LDA &0600,X | |
| AF0F | 153 000 006 | 99 00 06 | STA &0600,Y | |
| AF12 | 200 | C8 | INY | |
| AF13 | 232 | E8 | INX | |
| AF14 | * | 196 042 | C4 2A | CPY &2A |
| AF16 | 208 244 | D0 F4 | BNE -12 --> &AF0C | |
| AF18 | 6 | 132 054 | 84 36 | STY &36 |
| AF1A | ^ | 128 094 | 80 5E | BRA 94 --> &AF7A Exit with A = 0 |
