8-Bit Software

The BBC and Master Computer Public Domain Library

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9646 TRACE

Submitted by Steve Fewell

Description:

Call routine &9B1E to check if there is a tokenised Line Number following the TRACE Keyword. If a Line Number is
found then detokenise the Line Number and set the IWA to the Line Number value.

If the carry flag is not set, then a Line Number was not found and A contains the character after the TRACE Keyword, so:: * If A is the 'ON-Token' [#&EE] then jump to the TRACE ON routine at &9667; * If A is the 'OFF-Token' [#&87] then jump to the TRACE OFF routine at &9670; * Otherwise, the TRACE Line Number must be a variable/expression, so call routine &926F to evaluate; the variable/expression, convert the result to Integer and store the result in the IWA

Call routine &9BA6 to check that the statement terminates correctly with a ':', '<cr>' or 'ELSE-token' character,
issuing 'Syntax error' if it doesn't.
Set the TRACE Line Number (locations &21-&22) to the 16-bit Line Number from the IWA (locations &2A-&2B).
Set the TRACE flag (location &20) to #&FF to indicate that TRACEing is currently activated.

Jump to &9005 to continue executing the next statement of the Program.

[&9667] TRACE ON routine
Increment the BASIC Text Pointer A offset (&0A) to skip over the ON-Token.
Call routine &9BA6 to check that the statement terminates correctly with a ':', '<cr>' or 'ELSE-token' character,
issuing 'Syntax error' if it doesn't.
Set A to #&FF.

Jump to &965F to:: * Set the TRACE Line Number MSB (location &22) to A (#&FF) to indicate that there is no maximum; TRACE Line Number.; * Set the TRACE flag (location &20) to #&FF to indicate that TRACEing is currently activated.; * Jump to &9005 to continue executing the next statement of the Program.

[&9670] TRACE OFF routine
Increment the BASIC Text Pointer A offset (&0A) to skip over the OFF-Token.
Call routine &9BA6 to check that the statement terminates correctly with a ':', '<cr>' or 'ELSE-token' character,
issuing 'Syntax error' if it doesn't.
Set A to #&00.

Jump to &9663 to:: * Set the TRACE flag (location &20) to A (#&00) to indicate that TRACEing is NOT currently activated.; * Jump to &9005 to continue executing the next statement of the Program.

Disassembly for the TRACE routine

9646		032 030 155	20 1E 9B	JSR &9B1E Detokenise the Line Number at PTR A & Set IWA to the Line Number value
9649		176 011	B0 0B	BCS 11 --> &9656
964B		201 238	C9 EE	CMP#&EE
964D		240 024	F0 18	BEQ 24 --> &9667 TRACE ON
964F		201 135	C9 87	CMP#&87
9651		240 029	F0 1D	BEQ 29 --> &9670 TRACE OFF
9653	o	032 111 146	20 6F 92	JSR &926F Evaluate Expression at BASIC Text pointer A convert result to integer
9656		032 166 155	20 A6 9B	JSR &9BA6 Check end of Statement
9659	*	165 042	A5 2A	LDA &2A
965B	!	133 033	85 21	STA &21
965D	+	165 043	A5 2B	LDA &2B
965F	"	133 034	85 22	STA &22
9661		169 255	A9 FF	LDA#&FF
9663		133 032	85 20	STA &20
9665		128 212	80 D4	BRA -44 --> &963B [JMP &9005] Process the next statement
9667		230 010	E6 0A	INC &0A
9669		032 166 155	20 A6 9B	JSR &9BA6 Check end of Statement
966C		169 255	A9 FF	LDA#&FF
966E		208 239	D0 EF	BNE -17 --> &965F
9670		230 010	E6 0A	INC &0A
9672		032 166 155	20 A6 9B	JSR &9BA6 Check end of Statement
9675		169 000	A9 00	LDA#&00
9677		128 234	80 EA	BRA -22 --> &9663