After blinking the LEDs. I started with the chapter register and on the first page itself I got some issues. I read about most of them and find out that the address is const and to use that fixed value we used raw pointer GPIOE_BSRR as *mut u32 which is making a copy of the address(referencing) variable and allowing us to change.

[NitinSaxenait]

After blinking the LEDs. I started with the chapter register and on the first page itself I got some issues. I read about most of them and find out that the address is const and to use that fixed value we used raw pointer GPIOE_BSRR as *mut u32 which is making a copy of the address(referencing) variable and allowing us to change.
Second, we are dereferencing using * and putting the statement inside the unsafe block as per the raw pointer deference rules..

I hope till here i am getting it right now the next issue is what we are doing at right-hand side using the shift operator. We are using 9 11 to set and 16, 25 to reset but why 1 in every shift?

`#![no_main]
#![no_std]

#[allow(unused_imports)]
use aux7::{entry, iprint, iprintln};

#[entry]
fn main() -> ! {
aux7::init();

{
// A magic address!
const GPIOE_BSRR: u32 = 0x48001018;

    // Turn on the "North" LED (red)
    //we use the dereference operator * on a raw pointer that requires an unsafe block.
    //change data through the mutable pointer
    (GPIOE_BSRR as *mut u32) = 1 << 9;

    // Turn on the "East" LED (green)
    *(GPIOE_BSRR as *mut u32) = 1 << 11;

    // Turn off the "North" LED
    (GPIOE_BSRR as *mut u32) = 1 << (9 + 16);

    // Turn off the "East" LED
    (GPIOE_BSRR as *mut u32) = 1 << (11 + 16);
}
loop {}

}
`
@adamgreig

Originally posted by @NitinSaxenait in #193 (comment)

[adamgreig]

but why 1 in every shift?

It's a convenient way to express the full binary number we want to write to the register. The way the BSRR register works is you write a '1' to any of the 'set' fields (each 1 bit wide, bits 0-15) to turn on that GPIO, and write a '1' to any of the 'reset' fields (bits 16-31) to turn it off.

So, to turn on pin 2, you write a 1 to bit 2, which means writing 0b0000_0000_0000_0000_0000_0000_0000_0100 to the register. Instead of typing out the long number, we can write 1 << 2, which is the same number but easier to understand. We're shifting a 1 to the left by 2 places. Because all the fields can only take a '1', we never need any other number. To turn it off we'd need to write 0b0000_0000_0000_0100_0000_0000_0000_0000, or for short 1 << 18, but we write it 1 << (2 + 16) to make it clearer that it's the second bit, just shifted by an extra 16 positions to turn it off.

Sometimes a field is longer, for example it might be 4 bits starting at bit 5, and we want to write 0b1010 to that field: overall we'd need to write 0b0000_0000_0000_0000_0000_0001_0100_0000, but instead we could say 0b1010 << 5 or 6 << 5 which is the same thing.

Hope that helps answer your question!

[NitinSaxenait]

Yes thanks, @adamgreig for this. It is clear now.

[NitinSaxenait]

@adamgreig I have one more doubt. Do we still need to use the aux7 crate as if I remove it from the main program, the program doesn't work correctly. Can I consider LEDs as a room and to access them I need to go through the main door(device_peripheral)??