Spectrum ROM: Routine at 09525

Used by the routine at S_SCREEN.

This subroutine is used to find the character that appears at line x, column y of the screen. It only searches the character set 'pointed to' by CHARS.

Note: this is normally the characters 32 (space) to 127 (©) although the user can alter CHARS to match for other characters, including user-defined graphics.

Used by the routine at S_SCREEN. This subroutine is used to find the character that appears at line x, column y of the screen. It only searches the character set 'pointed to' by CHARS. Note: this is normally the characters 32 (space) to 127 (©) although the user can alter CHARS to match for other characters, including user-defined graphics.
S_SCRN_S	09525	CALL STK_TO_BC	x to C, y to B; 0<=x<=23; 0<=y<=31.
	09528	LD HL,(23606)	CHARS plus 256 gives HL pointing to the character set.
	09531	LD DE,256
	09534	ADD HL,DE
	09535	LD A,C	x is copied to A.
	09536	RRCA	The number 32*(x mod 8)+y is formed in A and copied to E. This is the low byte of the required screen address.
	09537	RRCA
	09538	RRCA
	09539	AND 224
	09541	XOR B
	09542	LD E,A
	09543	LD A,C	x is copied to A again.
	09544	AND 24	Now the number 64+8*INT (x/8) is inserted into D. DE now holds the screen address.
	09546	XOR 64
	09548	LD D,A
	09549	LD B,96	B counts the 96 characters.
S_SCRN_LP	09551	PUSH BC	Save the count.
	09552	PUSH DE	And the screen pointer.
	09553	PUSH HL	And the character set pointer.
	09554	LD A,(DE)	Get first row of screen character.
	09555	XOR (HL)	Match with row from character set.
	09556	JR Z,S_SC_MTCH	Jump if direct match found.
	09558	INC A	Now test for match with inverse character (get 0 in A from 255).
	09559	JR NZ,S_SCR_NXT	Jump if neither match found.
	09561	DEC A	Restore 255 to A.
S_SC_MTCH	09562	LD C,A	Inverse status (0 or 255) to C.
	09563	LD B,7	B counts through the other 7 rows.
S_SC_ROWS	09565	INC D	Move DE to next row (add 256).
	09566	INC HL	Move HL to next row (i.e. next byte).
	09567	LD A,(DE)	Get the screen row.
	09568	XOR (HL)	Match with row from the ROM.
	09569	XOR C	Include the inverse status.
	09570	JR NZ,S_SCR_NXT	Jump if row fails to match.
	09572	DJNZ S_SC_ROWS	Jump back till all rows done.
	09574	POP BC	Discard character set pointer.
	09575	POP BC	And screen pointer.
	09576	POP BC	Final count to BC.
	09577	LD A,128	Last character code in set plus one.
	09579	SUB B	A now holds required code.
	09580	LD BC,1	One space is now needed in the work space.
	09583	RST 48	Make the space.
	09584	LD (DE),A	Put the character into it.
	09585	JR S_SCR_STO	Jump to stack the character.
S_SCR_NXT	09587	POP HL	Restore character set pointer.
	09588	LD DE,8	Move it on 8 bytes, to the next character in the set.
	09591	ADD HL,DE	Move it on 8 bytes, to the next character in the set.
	09592	POP DE	Restore the screen pointer.
	09593	POP BC	And the counter.
	09594	DJNZ S_SCRN_LP	Loop back for the 96 characters.
	09596	LD C,B	Stack the empty string (length zero).
S_SCR_STO	09597	JP STK_STO	Jump to stack the matching character, or the null string if no match is found.
Note: this exit, via STK_STO, is a mistake as it leads to 'double storing' of the string result (see S_STRING). The instruction line should be 'RET'.