Self-modifying-code subroutine



EXAMP	EQU	$03	;Example table element (0 to N) to transfer from the
SIZE	EQU	$0E	;For this example, all elements have length 14 (base 10).
                        ;TABLE to TARGET

*********************************************************************************
*	Ram Table			*
*********************************************************************************
	ORG	$0050
SMCR@-{A}M	RMB	$4	;Self-modifying-code subroutine
TEMPX	RMB	$1	;Temp IDX storage
TEMPA	RMB	$1	;Temp ACC storage
TARGET	RMB	SIZE	;Target location of element length SIZE

*********************************************************************************
*	Initialization			*
*********************************************************************************
	ORG	$0400	;Start of rom
INIT	LDA	SMCROM	;Initialize the self-modifying-code.
	STA	SMCR@-{A}M	;LDA instruction
	LDA	SMCROM+$3
	STA	SMCR@-{A}M+$3	;RTS instruction

*********************************************************************************
*	Program start			*
*********************************************************************************
BEGIN	LDX	#EXAMP	;Transfer the example element
	JSR	XFER	;Transfer all bytes from the TABLE rom to TARGET ram
	STOP

*********************************************************************************
*	Subroutines			*
*********************************************************************************
*	Subroutine XFER
*	This routine transfers an element of the array TABLE(X) to TARGET.
*	The length of each element, in bytes, is SIZE (1<=SIZE<=255).
*	The number of the element to be transferred is in IDX.
*	Both ACC and IDX are returned intact.
XFER	STX	TEMPX	;Temporarily save IDX
	STA	TEMPA	;Temporarily save ACC
	LDA	#SIZE	;Calculate the effective address of the first
			;byte for the desired element.
	MUL
	ADD	SMCROM+$02	;Add the element offset to the TABLE offset
	STA	SMCR@-{A}M+$02
	TXA
	ADC	SMCROM+$01
	STA	SMCR@-{A}M+$01
	CLRX		;Begin transferring the individual bytes, starting with the
			;byte located at the effective address and ending with the byte
			;located at the effective address + SIZE -1
GETBYTE	JSR	SMCR@-{A}M	;Get the byte by extended addressing
	STA	TARGET,X	;Save in the TARGET ram
	INC	SMCR@-{A}M+$02	;Add 1 (double-precision) to the SMCR@-{A}M extended address
	BNE	NOINC
	INC	SMCR@-{A}M+$01
NOINC	INCX		;Update the index counter
	CPX	#SIZE	;Quit when SIZE bytes have been transferred
	BNE	GETBYTE
	LDX	TEMPX	;Restore IDX
	LDA	TEMPA	;Restore ACC
	RTS

*	Subroutine SMCROM (Dummy)
*	This dummy SMCROM (Self-modifying-code ROM) routine is transferred
*	to SMCR@-{A}M during program initialization. Extended address TABLE, the starting
*	address of the data table, is used to calculate an absolute address of a byte
*	of data in the table which is then transferred into ACC.
SMCROM	LDA	TABLE	;Absolute addressing pointing to the start of the
	RTS		;message table

*********************************************************************************
*	Data table			*
*********************************************************************************
*	This is the actual data table which can contain any number of entries (up to
*	the limits of the HC05's rom space), with all entries of length SIZE.
TABLE	FCB	'TABLE_ENTRY_00'
	FCB	'TABLE_ENTRY_01'
	FCB	'TABLE_ENTRY_02'
	FCB	'TABLE_ENTRY_03';Selected element for this example	
*	.
*	.
*	.
	FCB	'TABLE ENTRY 99'

*********************************************************************************
*	Vectors			*
*********************************************************************************
	ORG	$1FFE
RES	FDB	INIT	;Reset vector