Blink_LED.s [Exercise C]

; Blink_LED.s   Lab #1 sample program
; This program blink the LEDs on the DE0 board

; Stack related directives
    AREA    STACK, NOINIT, READWRITE, ALIGN=4   ; Name this block of code as STACK, reside in the RAM area
Stack_Size  EQU     0x200                       ; Stack Size = 0x200 bytes
Stack_Mem   SPACE   Stack_Size                  ; Reserve the space in RAM
TOP_STACK                                       ; Set top of stack location

; Vector Table - link to Address 0 at Reset
    AREA    RESET, DATA, READONLY               ; Name this block of code as RESET, reside in the ROM area
__Vectors   DCD     TOP_STACK                   ; Vector table start here, first enty is the 'Top of stack'
            DCD     START                       ; second entry is the Reset vector address

; User Application
    AREA    texts, CODE, READONLY               ; Name this block of code as texts, reside in the ROM area
ENTRY                                           ; Mark first instruction to execute
START       PROC                                ; Declaration of subroutine/function
            MOVS    R2,#0xA                     ; Store address 0xA0000000 in R2; start with 0xA
            LSLS    R2,R2,#28                   ; Logical left shift R2 to obtain the correct address-0xA0000000
            MOVS    R1,#0xA                     ; Store address 0xA0001000 in R3; start with 0xA **
            LSLS    R1,R1,#16                   ; R3 = 0xA0000 **
            ADDS    R1,#0x1                     ; R3 = 0xA0001 **
            LSLS    R1,R1,#12                   ; R3 = 0xA0001000 [Address of switches in R3] **
leds_odd    MOVS    R0,#0xAA                    ; R0 = 0000 1010 1010
            LSLS    R0,R0,#2                    ; R0 = 0010 1010 1000
            ADDS    R0,#0x02                    ; R0 = 0010 1010 1010
            LDR     R5,[R1]                     ; R4 = Mem[R3] **
            ORRS    R0,R0,R5                    ; Set Bits indicated by sliding switches **
            STR     R0,[R2,#0x00]               ; Store the value to the DE0_LED address
            BL      DELAY                       ; Call the DELAY function

leds_even   MOVS    R0,#0x55                    ; R0 = 0000 0101 0101
            LSLS    R0,R0,#2                    ; R0 = 0001 0101 0100
            ADDS    R0,#0x01                    ; R0 = 0001 0101 0101
            LDR     R5,[R1]                     ; R4 = Mem[R3] **
            ORRS    R0,R0,R5                    ; Set Bits indicated by sliding switches **
            STR     r0,[r2,#0x00]               ; Store the value to the DE0_LED address
            BL      DELAY                       ; Call the DELAY function
            B       leds_odd                    ; Repeat
            ENDP                                ; END of this subroutine

;Delay subroutine
COUNTER     EQU     0xFFFFF                     ; Counter to be used for delay looping

DELAY       PROC                                ; Declaration of DELAY subroutine
            MOVS    R4,#3
delay0      ldr     R3,=COUNTER                 ; Initialize R3 with delay COUNTER
delay1      SUBS    R3,#1
            BNE     delay1
            SUBS    R4,#1
            BNE     delay0
            BX      LR                          ;Return to the calling function
            ENDP



; The following codes are not used in this application, but their symbols are required for error-free linking by default linker setup
                EXPORT  __Vectors               ; default symbol required by the linker, not used in this program
                EXPORT  Reset_Handler           ; default symbol required by linker, not needed in this program

    AREA    texts, CODE, READONLY, ALIGN=4
Reset_Handler   PROC                            ; Typical code for Reset_vector handler
                LDR     R0, =START              ; User Application called by Label
                BX      R0
                ENDP

            END                                     ; End of code. Assembler ignore code beyind this point