Hi everyone,

I'm trying to port the SAM-BA applet project to work with IAR compiler instead of GCC.
Now, I have one assembly line that I don't know how to convert to IAR.

The relevant line is:
add r1, pc,#-(8+.-mailbox)

And the whole context is:
/* Branch to main()
******************/
mov r0, #1
add r1, pc,#-(8+.-mailbox)
ldr r3, =?main
mov lr, pc
bx r3

When the mailbox define as:
mailbox:
DS32 4*32

Thanks for your help!

Zohar

R0 is Param#1 = 1
R1 is Param#2 = &mailbox

It's the address of the mailbox, in a location independent fashion, so it doesn't matter where SAM-BA loads the applet, and fills the mailbox structure.

Something like this is viable in Keil, IAR would probably be similar. In Keil/GCC '.' is the assemblers program counter, the use of '*' or '$' is common in other assemblers.

here ADD R1, PC, #-(here + 8 - mailbox)

If IAR won't do the math for you, then you could always use a hard coded constant offset.

Hi CptTitanic,

Many thanks for fast reply.

Your explanation is clear to me except one thing:
What is "-(here + 8 - mailbox)"

I can't find any sense in that arithmetic...

Let's say mailbox adder is 0x301108 and PC on the relevant line is 0x301314. What then should be the second parameter of the main function?

Thanks in advance

Zohar
BTW, if this project will work I'll be happy to share it with the forum members.

The math is based on the fact the ARM is pipelined, the value read from PC in a computation will be 8 bytes ahead of the instruction that is executing.

When entering main(), R1 = 0x301108, the address is computed in an agnostic manner so that it doesn't matter what address the code is physically loaded or mapped. Other architectures would use 'LEA', Load Effective Address, but the ARM doesn't have that, so an equivalent is utilized.

00301108 mailbox
..
00301314 sub r1, pc, #0x214 ; r1 = pc - 0x214


-(0x301314 + 8 - 0x301108)
-(0x30131C - 0x301108)
-(0x214)

So PC + -(0x214) is better encoded as PC - 0x214, so the assembler typically will reverse the ADD and use the SUB variant automatically.