Assembly Directives

• Thumb - placed at the top of the file to specify that code is generated with Thumb instructions.
• CODE - Denotes the section for machine instructions (ROM).
• DATA Denotes the section for global variables(RAM).
• AREA -Instructs the assembler to assemble a new code or data section.
• SPACE Reserves a block of memory and fills it with zeros.
• ALIGN-Used to ensure next object aligns properly.
• EXPORT – to make an object accessible from another file
• GLOBAL – Same meaning as EXPORT
• IMPORT – to access an “exported” object
• END – Placed at the end of each file
• DCB – Places byte (8-bits) sized constant in memory
• DCW – Places a half-word (16-bits) sized constant into memory
• DCD – Places a word (32-bits) sized constant into memory
• EQU – To give a symbolic name to a numeric constant.
• |.text| Makes assembly code callable by C

        AREA    |.text|, CODE, READONLY, ALIGN=2
        THUMB
        EXPORT   __main

__main
        BL      GPIO_Init

Blink
        MOVW    R1, #LED_GREEN
        STR     R1, [R2]
        LDR     R3, =ONESEC
        BL      Delay

        MOVW    R1, #LED_BLUE
        STR     R1, [R2]
        LDR     R3, =HSEC
        BL      Delay

        MOVW    R1, #LED_RED
        STR     R1, [R2]
        LDR     R3, =HSEC
        BL      Delay

        MOVW    R1, #LED_ORANGE
        STR     R1, [R2]
        LDR     R3, =FSEC
        BL      Delay

        B       Blink

GPIO_Init
        LDR     R0, =RCC_AHB1ENR         ; R0 points to peripheral clock register
        LDR     R1, [R0]                 ; Load Peripheral clock register into R1
        ORR     R1, R1, GPIOIDEN         ; Enable GPIO port D clock
        STR     R1, [R0]                 ; Store result back

        LDR     R0, =GPIOD_MODER         ; R0 points to PORTD configuration register
        LDR     R1, =(MODER15_OUT | MODER14_OUT | MODER13_OUT | MODER12_OUT)
        STR     R1, [R0]                 ; Set pins 15,14,13,12 as output

        MOV     R1, #0
        LDR     R2, =GPIOD_ODR           ; GPIO output data register

        BX      LR

Delay
        SUBS    R3, R3, #1
        BNE     Delay
        BX      LR

        ALIGN

;-------------------------------
; Constants and Equates
;-------------------------------
RCC_BASE        EQU     0x40023800
RCC_AHB1ENR     EQU     0x40023830
GPIOD_BASE      EQU     0x40020C00
GPIOD_MODER     EQU     0x40020C00
GPIOD_ODR       EQU     0x40020C14

GPIOIDEN        EQU     1 << 3

MODER15_OUT     EQU     1 << 30
MODER14_OUT     EQU     1 << 28
MODER13_OUT     EQU     1 << 26
MODER12_OUT     EQU     1 << 24

LED_BLUE        EQU     1 << 15
LED_RED         EQU     1 << 14
LED_ORANGE      EQU     1 << 13
LED_GREEN       EQU     1 << 12

DELAY           EQU     0x000F
ONESEC          EQU     5333333
HSEC            EQU     266667
FSEC            EQU     106667

        END

MOV Instruction:

• Used to move data within the processor without accessing memory.
• Can only move 8-bit immediate values.