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Routines |
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The subroutine is entered with the HL register pair holding the base address of the thirty two bytes that form the pixel-line and the B register holding the pixel-line number.
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| COPY_LINE | 03828 | LD A,B | Copy the pixel-line number. | |||||||
| 03829 | CP 3 | The A register will hold 0 until the last two lines are being handled. | ||||||||
| 03831 | SBC A,A | |||||||||
| 03832 | AND 2 | |||||||||
| 03834 | OUT (251),A | Slow the motor for the last two pixel lines only. | ||||||||
| 03836 | LD D,A | The D register will hold either 0 or 2. | ||||||||
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There are three tests to be made before doing any 'printing'.
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| COPY_L_1 | 03837 | CALL BREAK_KEY | Jump forward unless the BREAK key is being pressed. | |||||||
| 03840 | JR C,COPY_L_2 | |||||||||
| 03842 | LD A,4 | But if it is then stop the motor, enable the maskable interrupt, clear the printer buffer and exit via the error handling routine - 'BREAK-CONT repeats'. | ||||||||
| 03844 | OUT (251),A | |||||||||
| 03846 | EI | |||||||||
| 03847 | CALL CLEAR_PRB | |||||||||
| 03850 | RST 8 | |||||||||
| 03851 | DEFB 12 | |||||||||
| COPY_L_2 | 03852 | IN A,(251) | Fetch the status of the printer. | |||||||
| 03854 | ADD A,A | |||||||||
| 03855 | RET M | Make an immediate return if the printer is not present. | ||||||||
| 03856 | JR NC,COPY_L_1 | Wait for the stylus. | ||||||||
| 03858 | LD C,32 | There are thirty two bytes. | ||||||||
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Now enter a loop to handle these bytes.
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| COPY_L_3 | 03860 | LD E,(HL) | Fetch a byte. | |||||||
| 03861 | INC HL | Update the pointer. | ||||||||
| 03862 | LD B,8 | Eight bits per byte. | ||||||||
| COPY_L_4 | 03864 | RL D | Move D left. | |||||||
| 03866 | RL E | Move each bit into the carry. | ||||||||
| 03868 | RR D | Move D back again, picking up the carry from E. | ||||||||
| COPY_L_5 | 03870 | IN A,(251) | Again fetch the status of the printer and wait for the signal from the encoder. | |||||||
| 03872 | RRA | |||||||||
| 03873 | JR NC,COPY_L_5 | |||||||||
| 03875 | LD A,D | Now go ahead and pass the 'bit' to the printer. Note: bit 2 low starts the motor, bit 1 high slows the motor, and bit 7 is high for the actual 'printing'. | ||||||||
| 03876 | OUT (251),A | |||||||||
| 03878 | DJNZ COPY_L_4 | 'Print' each bit. | ||||||||
| 03880 | DEC C | Decrease the byte counter. | ||||||||
| 03881 | JR NZ,COPY_L_3 | Jump back whilst there are still bytes; otherwise return. | ||||||||
| 03883 | RET | |||||||||
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