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Routines |
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Clear/reset input area (clear by printing 5 spaces):
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| 62201 | LD BC,3603 | Line number (B) = 14 Column number (C) = 19 |
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| 62204 | CALL 3545 | Call ROM 'CL-SET' routine HL register pair returned with screen display position to print at (18510) |
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| 62207 | CALL 62056 | Print the following characters on screen. On RETurn from this CALL, continue from 62216 | ||
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Characters to print - spaces to clear input area:
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| 62210 | DEFM " " | 5 spaces | ||
| 62215 | DEFB 255 | End-of-data byte | ||
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Prepare for player input:
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| 62216 | LD BC,3603 | Line number (B) = 14 Column number (C) = 19 |
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| 62219 | CALL 3545 | Call ROM 'CL-SET' routine HL register pair returned with screen display position to print at |
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| 62222 | LD B,4 | 4 digits to enter | ||
| 62224 | LD HL,0 | HL not needed for print routine (uses RST 16 instruction and stack pointer to the following data) | ||
| 62227 | CALL 62056 | Print the (following) characters on screen | ||
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Characters for flashing cursor:
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| 62230 | DEFM 18,1," ",18,0,8 | ASCII codes for: - FLASH 1 - " " (SPACE) - FLASH 0 - CURSOR LEFT |
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| 62236 | DEFB 255 | End-of-data marker | ||
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Check player input:
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| 62237 | BIT 5,(IY+1) | FLAGS (system variable) - bit 5 is set if a key is pressed | ||
| 62241 | JR Z,62237 | Keep checking until it's pressed | ||
| 62243 | RES 5,(IY+1) | Key pressed - reset bit 5 of FLAGS system variable | ||
| 62247 | LD A,(23560) | Get value of LAST-K (last pressed key) | ||
| 62250 | CP 12 | Has DELETE been pressed? | ||
| 62252 | JR Z,62201 | If so, jump back and reset the player's input (clear the input area) | ||
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Evaluate key pressed:
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| 62254 | LD C,A | Store key value in C register | ||
| 62255 | SUB 48 | Is code for key pressed <48 (number 0 = value of 48, the first valid character that can be typed) | ||
| 62257 | JR C,62237 | If so, it's not valid - jump back until another key is pressed | ||
| 62259 | CP 10 | Is the code for the key press one of the numbers (0-9)? | ||
| 62261 | JR NC,62237 | If not, it's not valid - jump back until another key is pressed | ||
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A valid key between 0 and 9 has been pressed
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| 62263 | LD E,A | Store value of key pressed (0-9) in E register | ||
| 62264 | LD A,C | Retrieve key code value from C register | ||
| 62265 | RST 16 | Print the value on screen | ||
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The value to check against is a 2 byte value stored at 61944. Therefore we need to multiple each accumulated figure/digit by 16 before adding the next:
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| 62266 | ADD HL,HL | HL = HL x2 | ||
| 62267 | ADD HL,HL | x4 | ||
| 62268 | ADD HL,HL | x8 | ||
| 62269 | ADD HL,HL | x16 | ||
| 62270 | LD D,0 | |||
| 62272 | ADD HL,DE | |||
| 62273 | DJNZ 62227 | Decrement the counter in the B register. Continue to check for input until all 4 digits have been entered | ||
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We now have the code value in the HL register pair - check it against the correct code:
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| 62275 | LD DE,(61944) | Get the correct code value | ||
| 62279 | XOR A | Clear the carry flag | ||
| 62280 | SBC HL,DE | Subtract the correct code from the user-entered code | ||
| 62282 | RET | Return to 62292 with zero flag result (Z = correct, NZ = incorrect) | ||
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