If you ask how pain it is for just using assembly language to turning on the led, I WOULD say very very very very....pain in the ass!!
Last year I was using STK200 which has atmega16 on it. I was able to display a simple digital clock by just using AVR assembly, which just took me several hours to write the code and display it on the Putty.
But I have to admit, Cortex-M3 is so different than that one. It would take several hours to review the ARM instructions and architecture. And probably it will still has compile error in the code.
Here's a little example:
GPIOC_CRL EQU 0x40011000
GPIOC_CRH EQU 0x40011004
GPIOC_IDR EQU 0x40011008
GPIOC_ODR EQU 0x4001100C
GPIOC_BSRR EQU 0x40011010
GPIOC_BRR EQU 0x40011014
GPIOC_LCKR EQU 0x40011018
;STACK_TOP EQU 0X20002000
AREA RESET,CODE,READONLY
;DCD STACK_TOP ;MSP pointer
DCD start
;reset
AREA RESET, CODE, READONLY
ENTRY
start
LDR r1, =GPIOC_CRH ; Address for port c control register
LDR r0, [r1]
STR r0, [r1] ; Write to contorl register
MOV32 r1, #GPIOC_ODR ; Address for port c output data register
MOV r0, #0x0A00 ; Value for port c
STR r0, [r1] ; Write value
loop
B loop
ALIGN
END
I ediited a little bit from the original version. You will know that it won't even debug step in for you since the crazy "MOVS R0,R0"....
You will never know how to deal with it unless you know this device, cortex-m3 will probably need to initialize the stack otherwise the debugger will not give a fxxx to you.
So if uncomment the two statements above in the code, the debugging will start working. I would said do not try using pure assembly in CORTEX-M4, that is a different device and more bugs waiting.
