The BBC and Master Computer Public Domain Library

BD92 Output character to screen

Submitted by Steve Fewell

Description:

There are various routines which output to the screen. Here is the main one used when printing to the screen.

If called from &BD8F then routine &BD6C is executed before contining to &BD92.

&BD6C outputs the 2-digit Hexadecimal number (stored in A), as follows:

Store A to the Stack.

Divide A by 16 (to move the top 4-bits of the byte to the lower half, and clearing the top 4-bits).

Call routine &BD77 to output the upper Hex digit as follows:

  * Add #&06 to A if A is greater than or equal to &0A.

  * Add #&30 to A (to turn A from a binary Hex digit to an ASCII character).

  * Store A to the stack.

  * If WIDTH (&23) is less than COUNT (&1E) then call &BA92 to output a new line.

  * Retrieve A from the stack.

  * Increment COUNT (&1E).

  * Jump to the Operating System's Write Character vector (&020E) to output the ASCII character in A.

Retrieve A from the Stack (now A = the original value of A when the routine was entered).

AND A with #&0F to clear the top 4-bits (as the Hex character these represent has already been output).

Continue to routine &BD77, which outputs the lower Hex digit as follows:

  * Add #&06 to A if A is greater than or equal to &0A.

  * Add #&30 to A (to turn A from a binary Hex digit to an ASCII character).

  * Store A to the stack.

  * If WIDTH (&23) is less than COUNT (&1E) then call &BA92 to output a new line.

  * Retrieve A from the stack.

  * Increment COUNT (&1E).

  * Jump to the Operating System's Write Character vector (&020E) to output the ASCII character in A.

If called from &BD92 then Set A to #&20 (Space character). This forces the character to output to be a space.

&BD94 Overview. This routine outputs the character with correct handling of the following:

* The COUNT variable

* New line when COUNT exceeds the WIDTH value

* Redirection of output (to memory) when *EDIT-mode is active

* New line when character to output is &0D ('<cr>')

If called from &BD94 then Check whether &1F (LISTO Flag) is negative (indicating '*EDIT' mode), if it is then
goto &BDA2; otherwise, con tinue to &BD98. In '*EDIT' mode any output to the screen is instead redirected to memory so
that '*EDIT' can pickup the text (program listing) and allow the user to edit it. If we are not in '*EDIT' mode then continue
to &BD98. &BDA2 stores the ASCII code in A to VARTOP (the memory location pointed to by (&02, &03)),
and then increments VARTOP LSB (&02). If the VARTOP LSB (&02) doesn't overflow then exit;
otherwise, increment &03 also, and do the following check (storing A temporarily to the stack while it is done): If VARTOP
MSB byte (&03) = HIMEM MSB byte (&07), goto &BD31 to call the BASIC Initialise routine (to reset, VARTOP, LOMEM,
HIMEM, etc...) & generate a 'No Room' error. Otherwise, exit the output routine (as we are not out of memory for '*EDIT' data).

If called from &BD98 and the ASCII character to output (in A) is #&0D (Carriage return) then send the character
to the screen (JSR &FFEE), zero COUNT (&1E) and exit.

Otherwise (&BD7F):

* Store the character on the stack.

* If WIDTH (&23) is less than COUNT (&1E) then call &BA92 to output a new line.

* Retrieve A from the stack.

* Increment COUNT (&1E).

* Jump to the Operating System's Write Character vector (&020E) to output the ASCII character in A to the screen.

* Exit the output routine.

BDB3 routine

If called from &BDB3 then the following is done:

Clear carry flag.

AND A with &1F to clear the top 3 bits.

If A now contains zero then exit.

If X is negative (top bit set) then exit.

Multiply X by 2 (shift the bits left 1 position).

If X is now zero then exit.

continue to &BDBF to output X number of spaces.

[&BDBF] Output X number of Spaces, as follows:

[&BDBF] Call routine &BD92 to output a space.

Decrement X.

if X is still positive then jump back to &BDBF.

Exit.

Note: Line breaks are automatically made as needed (as X can be a maximum of 255).

Disassembly for the Output Character to screen routine

BD6C	H	072	48	PHA
BD6D	J	074	4A	LSR A
BD6E	J	074	4A	LSR A
BD6F	J	074	4A	LSR A
BD70	J	074	4A	LSR A
BD71	w	032 119 189	20 77 BD	JSR &BD77
BD74	h	104	68	PLA
BD75	)	041 015	29 0F	AND#&0F
BD77		201 010	C9 0A	CMP#&0A
BD79		144 002	90 02	BCC 2 --> &BD7D
BD7B	i	105 006	69 06	ADC#&06
BD7D	i0	105 048	69 30	ADC#&30
BD7F	H	072	48	PHA
BD80	#	165 035	A5 23	LDA &23
BD82		197 030	C5 1E	CMP &1E
BD84		176 003	B0 03	BCS 3 --> &BD89
BD86		032 146 186	20 92 BA	JSR &BA92
BD89	h	104	68	PLA
BD8A		230 030	E6 1E	INC &1E
BD8C	l	108 014 002	6C 0E 02	JMP (&020E) WRCHV (OS Write Character vector)
BD8F	l	032 108 189	20 6C BD	JSR &BD6C
BD92		169 032	A9 20	LDA#&20
BD94	$	036 031	24 1F	BIT &1F
BD96	0	048 010	30 0A	BMI 10 --> &BDA2
BD98		201 013	C9 0D	CMP#&0D
BD9A		208 227	D0 E3	BNE -29 --> &BD7F
BD9C		032 238 255	20 EE FF	JSR &FFEE OSWRCH (Output the <cr> character to the screen)
BD9F	L	076 149 186	4C 95 BA	JMP &BA95 Zero COUNT (&1E) and exit routine
BDA2		146 002	92 02	STA (&02)
BDA4		230 002	E6 02	INC &02
BDA6		208 029	D0 1D	BNE 29 --> &BDC5
BDA8		230 003	E6 03	INC &03
BDAA	H	072	48	PHA
BDAB		165 003	A5 03	LDA &03
BDAD	E	069 007	45 07	EOR &07
BDAF		240 128	F0 80	BEQ -128 --> &BD31 Initialise page &7 etc... and generate 'No room' error
BDB1	h	104	68	PLA
BDB2	`	096	60	RTS
BDB3		024	18	CLC
BDB4	037 031	25 1F	AND &1F
BDB6		240 013	F0 0D	BEQ 13 --> &BDC5
BDB8		138	8A	TXA
BDB9	0	048 010	30 0A	BMI 10 --> &BDC5
BDBB	*	042	2A	ROL A
BDBC		170	AA	TAX
BDBD		240 006	F0 06	BEQ 6 --> &BDC5
BDBF		032 146 189	20 92 BD	JSR &BD92 Output a Space
BDC2		202	CA	DEX
BDC3		208 250	D0 FA	BNE -6 --> &BDBF
BDC5	`	096	60	RTS

Initialise page &7 (etc...) and generate 'No room' error

BD31		032 172 187	20 AC BB	JSR &BBAC Initialise Page 7 & reset Variable pointers, etc...
BD34	L	076 161 144	4C A1 90	JMP &90A1 No room error

 

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