// Write a subroutine to count the number of 'space' characters
// in a 256 byte buffer located in SRAM.
// Return the count in register r0.
// Note: In the ASCII character set, the code for a space is 0x20 (base 16).


.INCLUDE <m328pdef.inc>

.DSEG

buffer:  .BYTE   256      // 0 to 255
 
.CSEG
// Preload SRAM with string (indirect transfer from FPM to SRAM)
	ldi		ZH,high(mystring<<1) // set Z pointer at string address in Flash
	ldi		ZL,low(mystring<<1)
    ldi		YH,high(buffer)		 // set Y pointer at address 0x0100 in SRAM
	ldi		YL,low(buffer)

	ldi		r17,low(mystring<<1)	
	ldi		r16,low(stringend<<1)
	sub		r16,r17		// number of iterations in r16
	mov		r17,r16		// copy to r17 for next loop

preload:
	lpm		r5,Z+		// load character to r5
	st		Y+,r5		// store r5 in SRAM
	dec		r16
	brne	preload		// loop until string is loaded (48 bytes/characters)

	rcall   CountSpaces
end:
    rjmp   end

// Count Spaces
CountSpaces:
	clr		r0			// initialize r0 to 0
    ldi		YH,high(buffer) // set Y pointer at address 0x0100 for SRAM
	ldi		YL,low(buffer)
	ldi		r20,0x20	// load 0x20 to r20 for compare (ASCII space)
scan_buffer:		
    ld		r1,Y+	    // load at Z pointer, increment to next byte address
    cp		r1,r20	    // compare loaded byte to 0x20
    brne	nospace	    // if not 0x20, skip increment
    inc		r0		    // increment r0 if byte in buffer is 0x20
nospace:
	dec	    r17			// dec for outerloop
	brne	scan_buffer

	ret

.ORG 0x00E7
//  characters     0        1         2         3         4			5
//                 12345678901234567890123456789012345678901234567890123
//  blanks =           1      2    3    4  5      6  789  10 11  12    13 (0x0D)            
	mystring: .DB "This string will have 13 spaces as   it is wri tten. "
	stringend: .DB ""