Spectrum ROM | 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 +00 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 +00 or +02. | |||||
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. | |||||
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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