;
	; while loop, dividing 1024 repeatedly by 2, using Z80 assembly language
	;
	; Runs under CP/M 3.1 on YAZE-AG-2.51.2 Z80 emulator
	; Assembled with zsm4 on same emulator/OS, uses macro capabilities of said assembler
	; Created with vim under Windows
	;
	; Thanks to https://wikiti.brandonw.net for the idea for the conversion routine hl -> decimal ASCII
	;
	;
	; 2023-05-19 Xorph
	;

	;
	; Useful definitions
	;

	bdos	equ 05h		; Call to CP/M BDOS function
	strdel	equ 6eh		; Set string delimiter
	wrtstr	equ 09h		; Write string to console

	nul	equ 00h		; ASCII control characters
	cr	equ 0dh
	lf	equ 0ah

	cnull	equ '0'		; ASCII character constants

	;
	; Macros for BDOS calls
	;

setdel 	macro	char		; Set string delimiter to char
	ld	c,strdel
	ld	e,char
	call	bdos
	endm

print 	macro	msg		; Output string to console
	ld	c,wrtstr
	ld	de,msg
	call	bdos
	endm

newline	macro			; Print newline
	ld	c,wrtstr
	ld	de,crlf
	call	bdos
	endm

	;
	; =====================
	; Start of main program
	; =====================
	;

	cseg

	setdel	nul		; Set string delimiter to 00h
	ld	ix,value	; Register ix points to memory location of counter

while:
	ld	a,(ix)		; Z80 has no 16 bit compare, so we check the value byte by byte for 0
	or	a		; In contrast to other CPUs, loading a register does NOT set the flags
	jr	nz,docalc	; or-ing the accumulator with itself sets the flags and is faster than "cp 0"
	ld	a,(ix+1)
	or	a
	jr	z,endprog	; If both bytes are 0, end program - this jump could be optimized away
				; and replaced with a direct "ret z", but we want to simulate a "real"
				; while loop, so we continue (jump to) after the last loop statement

docalc:
	ld	hl,(value)	; Print the current value, followed by newline
	ld	iy,buffer	; Register iy points to memory location for current value as text for printout
	call	dispHL		; dispHL modifies iy, so it must be reset to the buffer on every iteration

	print	buffer
	newline

	srl	(ix+1)		; Neither has the Z80 a 16 bit shift operation for dividing by 2...
	rr	(ix)		; Shift the MSB of value right and then rotate the LSB with carry to the right

	jr	while		; Next iteration

endprog:
	ret			; Return to CP/M

	;
	; ===================
	; End of main program
	; ===================
	;

	;
	; Helper routines - notice that the Z80 does not have a divide instruction
	; Notice further that CP/M does not have any support for pretty-printing
	; formatted numbers and stuff like that. So we have to do all this by hand...
	;

	;
	; Converts the value (unsigned int) in register hl to its decimal representation
	; Register iy has memory address of target for converted value
	; String is terminated with nul character (\0)
	;

dispHL:
	ld	b,1		; Flag for leading '0'
	irp	x,<-10000,-1000,-100,-10,-1>
	ld	de,x		; Subtract powers of 10 and determine digit
	call	calcdig
	endm

	ld	a,nul		; Terminate result string with nul
	ld	(iy+0),a

	ret			; End of conversion routine

calcdig:
	ld	a,cnull-1	; Determine the digit character
incrdig:
	inc	a		; Start with '0'
	add	hl,de		; As long as subtraction is possible, increment digit character
	jr	c,incrdig

	sbc	hl,de		; If negative, undo last subtraction and continue with remainder
	cp	cnull		; Check for leading '0', these are ignored
	jr	nz,adddig
	bit	0,b		; Use bit instruction for check if flag set, register a contains digit
	ret	nz		; If '0' found and flag set, it is a leading '0' and we return
adddig:
	ld	b,0		; Reset flag for leading '0', we are now outputting digits
	ld	(iy+0),a	; Store character in memory and set iy to next location
	inc	iy

	ret			; End of conversion helper routine

	;
	; ================
	; Data definitions
	; ================
	;

	dseg

value:	defw	1024d		; Starting value for loop, 16 bit little endian
crlf:	defb	cr,lf,nul	; Generic newline
buffer:	defs	10		; Buffer for conversion of number to text