;EE315Ex02.s


; this is an EQU directive. It is for using labels instead of 
; actual hexadecimal memory locatons.

	; THUMB mean we are going to be using THUMB type ARM language. It is the language
	; we learn in this course
	THUMB
	; AREA reserves a memory location for us. 
	; "Processor please reserve a memory location for me. It will be a data memory,
	; i.e. I will put data in it (no code). 
	; And make a data alignmen for 2^2=4 bytes.
	; i.e. write my data starting from an adress location which is factor of 4. 
	AREA    |.data|, DATA, READWRITE, ALIGN=2
	; Create a global variable M. anybody, including the subroutines can reach it.
	; M will point the starting address of this memory location. 
	; and reserve 4 bytes startting from M.
	EXPORT Mem [DATA,SIZE=40];
	; now put zeros on these 40 bytes startng from M
Mem SPACE 40
	
	; reserve another AREA for me, in the CODE space (i.e. Instruction memory)
	; so what I will write after this point will be instructions, not data
	; make it read only so that no body writes over it. 
	; make data alignment of 2^2=4. Start wrting my code from a memory location, 
	; which is a factor of 4
  	AREA    |.text|, CODE, READONLY, ALIGN=2
	; make Start label, which is the beginning memory location of my instructions.
	; so that I may call it from another file, or subroutine, (again make it global)
    EXPORT  Start

Start
	LDR R2,=Mem
	; now let's write an 10x16bit array to Mem
	; the array is [101 98 85 91 121 103 93 84 86 102]
	LDR R0,=101
	STRH R0,[R2]
	
	LDR R0,=98
	STRH R0,[R2,#2]
	
	LDR R0,=85
	STRH R0,[R2,#4]
	
	LDR R0,=91
	STRH R0,[R2,#6]
	
	LDR R0,=121
	STRH R0,[R2,#8]
	
	LDR R0,=103
	STRH R0,[R2,#10]
	
	LDR R0,=93
	STRH R0,[R2,#12]
	
	LDR R0,=84
	STRH R0,[R2,#14]
	
	LDR R0,=86
	STRH R0,[R2,#16]
	
	LDR R0,=102
	STRH R0,[R2,#18]

	; check sum of 10 elements starting from Mem at R0;
	; the result should be 0x3C4 (964)
	BL SumMem

	; check the sum of squares of 10 elements of Mem at R0
	; the results is 0X16f72 (94066)
	BL SumSquaresMem
	
theend
	B theend
	; this is the end of the main code
	
	
	;We have the subroutine here
SumMem
	; let's first calcaulte the mean
	LDR R2,=Mem
	MOV R0,#0	;initialise R0 to 0
	MOV R3,#0	;init the counter
	MOV R7,#10	; how many elements
loop1
	; calcaulte the mean
	LSL R3,#1
	LDRH R1,[R2,R3]
	ADD R0,R1
	LSR R3,#1
	ADD R3,#1
	CMP R3,R7
	BNE loop1
	BX LR
	
	;We have the subroutine here
SumSquaresMem
	; let's first calcaulte the mean
	LDR R2,=Mem
	MOV R0,#0	;initialise R0 to 0
	MOV R3,#0	;init the counter
	MOV R7,#10	; how many elements
loop2
	; calcaulte the mean
	LSL R3,#1
	LDRH R1,[R2,R3]
	MUL R1,R1
	ADD R0,R1
	LSR R3,#1
	ADD R3,#1
	CMP R3,R7
	BNE loop2
	BX LR


	ALIGN              	; This is a directive, not an instruction.
						; I wont write any ore code or data. but if I decide to write, you better stay aligned.
 	END					; we are really done this time.