h1. ARM7 Hello World Collection (in Progress)

Here are a collection of ways to implement hello world in arm7 assembler(gnu gas). This one uses the pseudo-instruction ‘ADR’ to load the address of a label into r0. ADR is a short range instruction - the compiler will error if it cannot reach the instruction. Depending on the thumb version as to the pc (program counter) relative offset of the address.

bc.. .cpu arm7tdmi .text .global main .type main, %function main: bl consoleDemoInit adr r0, .LhelloStr bl printf .L2: bl swiWaitForVBlank b .L2 .LhelloStr: .ascii "Hello World!0000"

p. This version turns the necessary padding into a .space directive

bc.. .cpu arm7tdmi .text .global main .type main, %function main: bl consoleDemoInit adr r0, .LhelloStr bl printf .L2: bl swiWaitForVBlank b .L2 .LhelloStr: .ascii "Hello World!00" .space 1

p. In this example we switch to the .string directive to specify a null terminated string

bc.. .cpu arm7tdmi .text .global main .type main, %function main: bl consoleDemoInit adr r0, .LhelloStr bl printf .L2: bl swiWaitForVBlank b .L2 .LhelloStr: .string "Hello World!" .space 1

p. In this version we use .align directive instead of space. This ensures alignment to the next 2^1 byte boundary

bc.. .cpu arm7tdmi .text .global main .type main, %function main: bl consoleDemoInit adr r0, .LhelloStr bl printf .L2: bl swiWaitForVBlank b .L2 .LhelloStr: .string "Hello World!" .align 1

p. This one uses ldr to insert a value at a label that signifies the address of another label. In this example we only get success when we ensure that the alignment is on the 2^2 byte boundary. I am still trying to understand why.

bc.. .cpu arm7tdmi .text .global main .type main, %function main: bl consoleDemoInit ldr r0, .Lhello bl printf .L2: bl swiWaitForVBlank b .L2 .LhelloStr: .string "Hello World!" .align 2 .Lhello: .word .LhelloStr

p. This one puts the string into the ROM and loads via LDR

bc.. .cpu arm7tdmi .section .rodata .LhelloStr: .string "Hello World!" .text .global main .type main, %function main: bl consoleDemoInit ldr r0, .Lhello bl printf .L2: bl swiWaitForVBlank b .L2 .Lhello: .word .LhelloStr

p. This one stores the string in a data section and loads via LDR

bc.. .cpu arm7tdmi .data .LhelloStr: .string "Hello World!" .text .global main .type main, %function main: bl consoleDemoInit ldr r0, .Lhello bl printf .L2: bl swiWaitForVBlank b .L2 .Lhello: .word .LhelloStr

p. [email protected]