Spectrum ROM: Routine at 2535

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 +20 (space) to +7F (©) 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 +20 (space) to +7F (©) although the user can alter CHARS to match for other characters, including user-defined graphics.
S_SCRN_S	2535	CALL STK_TO_BC	x to C, y to B; 0<=x<=23; 0<=y<=31.
	2538	LD HL,($5C36)	CHARS plus +0100 gives HL pointing to the character set.
	253B	LD DE,$0100
	253E	ADD HL,DE
	253F	LD A,C	x is copied to A.
	2540	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.
	2541	RRCA
	2542	RRCA
	2543	AND $E0
	2545	XOR B
	2546	LD E,A
	2547	LD A,C	x is copied to A again.
	2548	AND $18	Now the number 64+8*INT (x/8) is inserted into D. DE now holds the screen address.
	254A	XOR $40
	254C	LD D,A
	254D	LD B,$60	B counts the 96 characters.
S_SCRN_LP	254F	PUSH BC	Save the count.
	2550	PUSH DE	And the screen pointer.
	2551	PUSH HL	And the character set pointer.
	2552	LD A,(DE)	Get first row of screen character.
	2553	XOR (HL)	Match with row from character set.
	2554	JR Z,S_SC_MTCH	Jump if direct match found.
	2556	INC A	Now test for match with inverse character (get +00 in A from +FF).
	2557	JR NZ,S_SCR_NXT	Jump if neither match found.
	2559	DEC A	Restore +FF to A.
S_SC_MTCH	255A	LD C,A	Inverse status (+00 or +FF) to C.
	255B	LD B,$07	B counts through the other 7 rows.
S_SC_ROWS	255D	INC D	Move DE to next row (add +0100).
	255E	INC HL	Move HL to next row (i.e. next byte).
	255F	LD A,(DE)	Get the screen row.
	2560	XOR (HL)	Match with row from the ROM.
	2561	XOR C	Include the inverse status.
	2562	JR NZ,S_SCR_NXT	Jump if row fails to match.
	2564	DJNZ S_SC_ROWS	Jump back till all rows done.
	2566	POP BC	Discard character set pointer.
	2567	POP BC	And screen pointer.
	2568	POP BC	Final count to BC.
	2569	LD A,$80	Last character code in set plus one.
	256B	SUB B	A now holds required code.
	256C	LD BC,$0001	One space is now needed in the work space.
	256F	RST $30	Make the space.
	2570	LD (DE),A	Put the character into it.
	2571	JR S_SCR_STO	Jump to stack the character.
S_SCR_NXT	2573	POP HL	Restore character set pointer.
	2574	LD DE,$0008	Move it on 8 bytes, to the next character in the set.
	2577	ADD HL,DE	Move it on 8 bytes, to the next character in the set.
	2578	POP DE	Restore the screen pointer.
	2579	POP BC	And the counter.
	257A	DJNZ S_SCRN_LP	Loop back for the 96 characters.
	257C	LD C,B	Stack the empty string (length zero).
S_SCR_STO	257D	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'.