The BBC and Master Computer Public Domain Library

92BE CALL

Submitted by Steve Fewell

Description:

Call routine &9D2F to get the result of the expression at the BASIC Text pointer A location. This value should be the
CALL address.
Call routine &96BC to convert the result to an Integer value (if Float, or issue 'Type mismatch' error if it is a
String value).

Push the Integer call address value (IWA) to the BASIC stack.
Store zero in location &0600 (the parameter block used to pass parameters to the machine code routine - &0600
contains the number of variables in the parameter block, which is initalised to 0).
Set Y to 0 (the last used location in the machine code call parameter block).
[&92CC] Store Y on the 6502 stack.
Get the next non-space character from BASIC text pointer B location.

If the next character was a comma ',', a (or another) parameter value is present (and is required to be stored in the
machine code call parameter block (&0600). This is done as follows:

* Set Y to &1B (the BASIC Text pointer B offset)

* Call routine &9901 to store Y to &1B (Text pointer B offset), skip any space characters and return the

    address or the value of the variable denoted by the variable name (located after the comma)

* If routine &9901 returns with the zero flag set then issue 'No such variable' error as a variable name was not

    found

* Retrieve Y from the 6502 stack, and increment Y (to point to the next free location in the parameter block)

* Store the variable address (from location &2A-&2B) and type (from location &2C) details in the next 3

    locations in the machine code call parameter block (starting at offset Y) and increment Y twice

* Increment &0600 (the number of variables in the machine code call parameter block)

* Jump back to &92CC to check for further variable parameters

[&92F1] Otherwise (no comma found), there are no (or no further) variable paremeter details to store in the machine
code call parameter block (located at &0600-&06FF).

Retrieve Y from the 6502 stack (it's value can now be discarded).
Decrement the BASIC Text pointer B offset (&1B) to point to the character after the CALL address (or after the last
variable parameter value) and call routine &9B96 to check that the statement is terminated correctly with either a
':', '<cr>' or 'ELSE-token' character ('Syntax error' is issued if the statement is not correctly terminated).

Retrieve the CALL Address value from the BASIC stack (and store the value in the IWA).

Call the &9304 subroutine to execute the machinbe code routine as follows:

* Set A to &040C (the LSB of the C% variable) and shift the bits right 1 place, so that the carry flag is

    set to the value of the first bit (bit 0) of the &040C value.

* Set A to &0404 (the LSB byte of the A% variable)

* Set X to &0460 (the LSB byte of the X% variable)

* Set Y to &0464 (the LSB byte of the Y% variable)

* jump to the machine code address pointed to by IWA bytes &2A-&2B. The machine code routine will provide

    the return instruction to return from the &9304 subroutine.

[&92FD] After the routine &9304 has been executed, and the machine code routine has been run, the decimal flag
is cleared (as BASIC requires the decimal flag to be clear - otherwise its calculations will be incorrect) and jumps
to &9005 to execute the next statement in the BASIC program/command line input.

Disassembly for the CALL routine

92BE	/	032 047 157	20 2F 9D	JSR &9D2F Ptr B = Ptr A & Get result of expression
92C1		032 188 150	20 BC 96	JSR &96BC Check result type (location &27) - if Float then convert to Integer
92C4	&	032 038 188	20 26 BC	JSR &BC26 Push IWA value to the BASIC Stack [pushi]
92C7		156 000 006	9C 00 06	STZ &0600
92CA		160 000	A0 00	LDY#&00
92CC	Z	090	5A	PHY
92CD		032 235 142	20 EB 8E	JSR &8EEB Get next non-space char (PTR B) & compare with ','
92D0		208 031	D0 1F	BNE 31 --> &92F1
92D2		164 027	A4 1B	LDY &1B
92D4		032 001 153	20 01 99	JSR &9901 Store Y to &1B, skip spaces & Evaluate variable/array name & return the address of the value
92D7	(	240 040	F0 28	BEQ 40 --> &9301   'No such variable' error
92D9	z	122	7A	PLY
92DA		200	C8	INY
92DB	*	165 042	A5 2A	LDA &2A
92DD		153 000 006	99 00 06	STA &0600,Y
92E0		200	C8	INY
92E1	+	165 043	A5 2B	LDA &2B
92E3		153 000 006	99 00 06	STA &0600,Y
92E6		200	C8	INY
92E7	,	165 044	A5 2C	LDA &2C
92E9		153 000 006	99 00 06	STA &0600,Y
92EC		238 000 006	EE 00 06	INC &0600
92EF		128 219	80 DB	BRA -37 --> &92CC
92F1	z	122	7A	PLY
92F2		198 027	C6 1B	DEC &1B
92F4		032 150 155	20 96 9B	JSR &9B96 Check for end of Statement (PTR B)
92F7		032 230 188	20 E6 BC	JSR &BCE6 Retrieve IWA value from the BASIC Stack [popi]
92FA		032 004 147	20 04 93	JSR &9304 Execute the machine code routine pointed to by the IWA (&2A-&2B)
92FD		216	D8	CLD
92FE	L	076 005 144	4C 05 90	JMP &9005 Continue running program (execute next statement)
9301	L	076 140 173	4C 8C AD	BNE 12 --> &AD8C No such variable error
9304		173 012 004	AD 0C 04	LDA &040C
9307	J	074	4A	LSR A
9308		173 004 004	AD 04 04	LDA &0404
930B	`	174 096 004	AE 60 04	LDX &0460
930E	d	172 100 004	AC 64 04	LDY &0464
9311	l*	108 042 000	6C 2A 00	JMP (&002A)

 

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