The BBC and Master Computer Public Domain Library

A085 Print Line Number on screen

Submitted by Steve Fewell

Routine:Print Line Number
Name: Print Line number (&2A-&2B) on the screen
Starting Address: &A085
Entry criteria: &2A and &2B (the IWA) contain an untokenised Line Number.

Description:

This routine uses bytes stored at &8021-&802A, which contain the five 16-bit values: 1, 10, 100, 1000 and 10000.
These values are used to print Line Numbers (which are a maximum of 5 decimal digits wide).
The tens conversion table (&8021-&802A) consists of the following:

High byte Location	Value	Low Byte Location	Value	Description
8021	&00	8026	&01	The high and low byte of the value &0001 (1 in decimal)
8022	&00	8027	&0A	The high and low byte of the value &000A (10 in decimal)
8023	&00	8028	&64	The high and low byte of the value &0064 (100 in decimal)
8024	&03	8029	&E8	The high and low byte of the value &03E8 (1000 in decimal)
8025	&27	802A	&10	The high and low byte of the value &2710 (10000 in decimal)

Firstly, this routine sets the number of bytes in the Print output field (location &14).
If this routine is entered at location &A085 (i.e. from LIST) then the Print output field will be set to 5-digits
wide - so that all Line Numbers are positioned to 5 characters wide & therefore line up, as the following example shows:

    1
    5
   10
   70
  110
 2000
 2010
11100

If this routine is entered at location &A081 (i.e. from TRACE line printout) then the Print output field will be set to 0
(no fixed width) - so that all Line Numbers are positioned to 0 characters wide (and will take up as many characters as
needed), the following is an example of this output width, using a selection of different Line Numbers:

1
5
10
70
110
2000
2010
11100

Locations &3F-&43 are used to store the units/tens values (digit counts) as follows:
   - Location &3F is the number of 1's units in the Line Number (i.e. the value of the digit in the unit position).
   - Location &40 is the number of 10's units in the Line Number.
   - Location &41 is the number of 100's units in the Line Number.
   - Location &42 is the number of 1000's units in the Line Number.
   - Location &43 is the number of 10000's units in the Line Number.

Set X to #&04 (as the number of values to process is 5). The largest value (10000) will be processed first, followed by
the second largest (1000), and then the next largest (100), and then 10 and finally 1.

[&A08B] Zero the digit count for the current search value. The search value is the value pointed to by the value in X.
I.e. when X is 4, the search value's digit count store is location &3F + #&04 = &43, the search value MSB
byte is located at location &8021 + #&04 = &8025, and the search value LSB byte is located at location &8026
+ #&04 = &802A.
E.g. So, when X is 3 we will search for the value 1000, and location &42 will be zeroed at the start of the search; or
when X is 1 we will search for the value 10, and location &40 will be zeroed at the start of the search.

Set the carry flag to begin a subtraction. Note: the carry flag is not reset during subtractions of the same Search Value,
as the carry is carried over between subtractions of the same Search Value (where X is unchanged).
[&A08E] Subtract the current Search Number low byte (&8026 + X) from the low byte of the Line Number in the IWA (&2A).
Store the result in Y.
Subtract the current Search Number high byte (&8021 + X) from the high byte of the Line Number in the IWA (&2B).
Store the result in A.
If the subtraction did not underflow (carry is set), then the search value was sucessfully subtracted; so store the new Line
Number value (after the subtraction -> Y = low byte, A = high byte) in the IWA (locatons &2A, &2B respecively),
increment the Digit Counter (&3F + X) and jump back to &A08E to try another subtraction of the Search value.

Otherwise [&A0A3], the Search Value could not be subtracted, so we need to move on to the next (lower) search value).
Decrement X (to move down to the next lower Search value).
If X is positive (>=0) then jump back to &A08B to check whether we can subject this Search value from the IWA value.

Otherwise, the search has been completed, and locations &3F-&43 should now contain the separate digits of the line
Number - ready to print.

Find the first non-zero digit Set X to 5 (possible number of bytes to output).
[&A0A8] decrement X (and if X is zero then jump to the output routine - &A0AF) and check whether the Digit Counter
value at location &3F + X is zero. if it is then jump back to &A0A8 to find the first non-zero digit.

Output the Line Number value to the screen Now X points to the first non-zero byte (or the first byte if the Line Number is zero), where X=4 points to the digit at
location &43 and X=0 points to the digit at location &3F.
Store X in location &37 (this is a pointer to the first digit required to be output).

Print any required leading spaces, as follows:
If the Output field width value (location &14) is not zero, then subtract &37 (X) from the &14 value, the result
is the number of spaces required to be output on the line before the first non-zero digit is output.
If the number of spaces required is not zero then set X to the number of spaces required and call routine &BDBF output
X number of spaces.

Set X to the first digit to output pointer (from location &37).
Print each digit of the line number to the screen (starting at location &3F + X and finishing at location &3F,
reducing X by 1 after each digit has been printed.
To print each digit, the decimal value is ORA-ed with #&30 to add the ASCII offset value (64).
Exit after all required digits have been output.

Disassembly for the Print Line Number on screen routine

A081		169 000	A9 00	LDA#&00
A083		128 002	80 02	BRA 2 --> &A087
A085		169 005	A9 05	LDA#&05
A087		133 020	85 14	STA &14
A089		162 004	A2 04	LDX#&04
A08B	t?	116 063	74 3F	STZ &3F,X
A08D	8	056	38	SEC
A08E	*	165 042	A5 2A	LDA &2A
A090	&	253 038 128	FD 26 80	SBC &8026,X
A093		168	A8	TAY
A094	+	165 043	A5 2B	LDA &2B
A096	!	253 033 128	FD 21 80	SBC &8021,X
A099		144 008	90 08	BCC 8 --> &A0A3
A09B	+	133 043	85 2B	STA &2B
A09D	*	132 042	84 2A	STY &2A
A09F	?	246 063	F6 3F	INC &3F,X
A0A1		128 235	80 EB	BRA -21 --> &A08E
A0A3		202	CA	DEX
A0A4		016 229	10 E5	BPL -27 --> &A08B
A0A6		162 005	A2 05	LDX#&05
A0A8		202	CA	DEX
A0A9		240 004	F0 04	BEQ 4 --> &A0AF
A0AB	?	181 063	B5 3F	LDA &3F,X
A0AD		240 249	F0 F9	BEQ -7 --> &A0A8
A0AF	7	134 055	86 37	STX &37
A0B1		165 020	A5 14	LDA &14
A0B3		240 010	F0 0A	BEQ 10 --> &A0BF
A0B5	7	229 055	E5 37	SBC &37
A0B7		240 006	F0 06	BEQ 6 --> &A0BF
A0B9		170	AA	TAX
A0BA		032 191 189	20 BF BD	JSR &BDBF Output X number of Spaces
A0BD	7	166 055	A6 37	LDX &37
A0BF	?	181 063	B5 3F	LDA &3F,X
A0C1	0	009 048	09 30	ORA#&30
A0C3		032 148 189	20 94 BD	JSR &BD94 Output character to the screen
A0C6		202	CA	DEX
A0C7		016 246	10 F6	BPL -10 --> &A0BF
A0C9	`	096	60	RTS

 

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