; Convert intext to upper case with some OS 'flavor'

	JMP UCstart

intext:	DB "In Text !@#" ; Variable
	DB 0xFF	; String terminator
ucA:	DB 'A' ; upper case A value
ucZ:	DB 'Z' ; upper case Z value
lca:	DB 'a' ; lower case a value
lcz:	DB 'z' ; lower case z value
lcuc:	DB 0d ; Will hold lc to uc difference
; not needed outbuf:	DB 0xE8 ; output buffer start memloc

UCstart:
; Calculate the difference between lca and ucA values
; Store result in lcuc
	MOV A, [ucA]
	MOV B, [lca]
	SUB B, A
	MOV [lcuc], B

; For remainder
;	reg C has the memloc of the current char
;	reg B has the current output offset
;	reg A has the current char ASCII value

	MOV C, intext ; Get 1st char of intext memloc
	MOV B, 0x00 ; Set output buffer offset to 0

loopch:	MOV A, [C] ; Get next intext char value
	CMP A, 0xFF ; Check if string terminator
	JNE convch ; If not convert the character
	HLT ; normal termination (i.e., string terminator)

convch:	CMP A, [lca] ; Is char >= lc a?
	JB showch ; If not show the char
	CMP A, [lcz] ; Is char <= lc z?
	JA showch ; If not show the char

	SUB A, [lcuc] ; Subtract to convert to upper case

showch: MOV [OBchar], A ; 'Pass' the ASCII character
	MOV [OBloc], B ; 'Pass' the buffer offset	
	CALL OBch ; Have the 'OS' output the character

	INC B ; increment output buffer memloc
	INC C ; increment intext memloc
	JMP loopch ; process next character

; OS code

OBbeg:	DB 0xE8 ; Output buffer begin location
OBend:	DB 0xFF ; Output buffer end location
OBchar:	DB 0x00 ; Passed ASCII character to output
OBloc:	DB 0x00 ; Passed output buffer offset
 
OBch:
	PUSH A ; preserve reg A value
	PUSH B ; preserve reg B value

	MOV A, [OBchar] ; get the 'passed' char in reg A
	MOV B, [OBloc] ; get the 'passed' buffer offset
	ADD B, [OBbeg] ; Calc output buffer location

	CMP B, [OBbeg] ; Check if location >= OB begin loc
	JB OBErr ; If not jump to error

	CMP B, [OBend] ; Check if location <= OB end loc
	JA OBErr ; if not jump to error

	MOV [B], A ; put character into output buffer
	POP B ; restore reg B
	POP A ; restore reg A
	RET

OBErr:	; Invalid output attempted
; a 'real' OS would have the app exit with an error code
	MOV B, [OBbeg] ; Get loc of output buffer
	MOV [B], '*' ; put an * to indicate an issue
	HLT ; error termination