The Hobbit (v1.0): Routine at 7B15

The Hobbit can send its output to a ZX Printer (the PRINT and NOPRINT commands). If the flag at $B5E3 is set, this routine is called by 7694 at the end of every line of text, and it copies the 8 pixel rows of that line from the screen, starting at $5020, to the printer.

The ZX Printer is driven directly through port $FB. For each pixel row, the 32 bytes are sent one bit at a time: for each dot the routine waits for the printer's encoder signal (bit 0 of the port) and then outputs the dot. D controls the motor speed: it starts at 1 (slow), and after the first two rows the value derived from E runs the motor at full speed. After the 8 rows the motor is stopped (OUT ($FB),4). If the printer is not connected (bit 7 of the port reads as 1) the routine gives up straight away.

The Hobbit can send its output to a ZX Printer (the PRINT and NOPRINT commands). If the flag at $B5E3 is set, this routine is called by 7694 at the end of every line of text, and it copies the 8 pixel rows of that line from the screen, starting at $5020, to the printer. The ZX Printer is driven directly through port $FB. For each pixel row, the 32 bytes are sent one bit at a time: for each dot the routine waits for the printer's encoder signal (bit 0 of the port) and then outputs the dot. D controls the motor speed: it starts at 1 (slow), and after the first two rows the value derived from E runs the motor at full speed. After the 8 rows the motor is stopped (OUT ($FB),4). If the printer is not connected (bit 7 of the port reads as 1) the routine gives up straight away.
7B15	LD A,($B5E3)	Is the printer switched on?
7B18	AND A	Is A zero?
7B19	RET Z	Return if it is zero
7B1A	PUSH HL	Save HL
7B1B	PUSH DE	Save DE
7B1C	PUSH BC	Save BC
7B1D	LD D,$01	Slow motor; HL = start of the line on the screen
7B1F	LD HL,$5020	HL = $5020
7B22	XOR A	A = 0
7B23	LD E,A	E = A
7B24	OUT ($FB),A	Start the motor and wait for the printer to be ready
7B26	IN A,($FB)	Read the keyboard/port
7B28	ADD A,A	Add A,A
7B29	JP M,$7B67	Bit 7 set: no printer connected
7B2C	JR NC,$7B26	If carry clear, go to 7B26
7B2E	PUSH HL	Save HL
7B2F	PUSH DE	Save DE
7B30	LD A,D	A = D
7B31	CP $02	Compare with 2
7B33	SBC A,A	Subtract A,A with carry
7B34	AND E	Keep only the bits of E
7B35	RLCA	Rotate A left
7B36	AND E	Keep only the bits of E
7B37	LD D,A	D = A
7B38	LD C,(HL)	C = (HL)
7B39	PUSH HL	Save HL
7B3A	LD B,$08	Send eight dots per screen byte
7B3C	LD A,D	A = D
7B3D	RLC C	Shift/rotate C
7B3F	RRA	Rotate A right through carry
7B40	LD H,A	H = A
7B41	IN A,($FB)	Wait for the encoder...
7B43	RRA	Rotate A right through carry
7B44	JR NC,$7B41	If carry clear, go to 7B41
7B46	LD A,H	A = H
7B47	OUT ($FB),A	...and send the dot
7B49	DJNZ $7B3C	Loop back until B reaches 0
7B4B	POP HL	Restore HL
7B4C	INC HL	Next byte, until all 32 have been done
7B4D	LD A,L	A = L
7B4E	AND $1F	Keep only the bits of $1F
7B50	JR NZ,$7B38	If it is not zero, go to 7B38
7B52	IN A,($FB)	Read the keyboard/port
7B54	RRA	Rotate A right through carry
7B55	JR NC,$7B52	If carry clear, go to 7B52
7B57	LD A,D	A = D
7B58	RRCA	Rotate A right
7B59	OUT ($FB),A	Write to the port (border colour, beeper or printer)
7B5B	POP DE	Restore DE
7B5C	POP HL	Restore HL
7B5D	INC H	Next pixel row
7B5E	INC E	Increment E
7B5F	BIT 3,E	Test bit 3 of E
7B61	JR Z,$7B24	If the bit is clear, go to 7B24
7B63	LD A,$04	Stop the motor
7B65	OUT ($FB),A	Write to the port (border colour, beeper or printer)
7B67	POP BC	Restore BC
7B68	POP DE	Restore DE
7B69	POP HL	Restore HL
7B6A	RET	Done