Bootloader of the main program. Exception handling. Output data to the debug port or console

Learn how to work with RAM, use special instructions, use Cortex-M4 processor shutdowns. Creating a minimum system bootloader. Learn how to use data output through a debug port (or console).

📝 Task

1. Understand how bootloader, exception handling, and semihosting should work.
2. Modify the expression calculation file from the Lab Work 2 (insert a vector table into it), and modify Makefile so that <your_file_name>.bin file is created automatically.
3. Create a bootloader that will run the program (for example, kernel.bin).
4. Run the program in gdb, demonstrate the launch of the loaded program and output the result to the console.

Commands for working with memory	Increment/Decrement of the address register	Type of shift	Number of bytes to shift
LDR, STR	decrement	register	4

📙 Description

The purpose of this laboratory work is to develop a primitive program loader, ie to build a program that can byte-by-byte another program into RAM and run it from there.Consider this program in more detail on the example:

Content of kernel.S (with comments)

.syntax unified
.cpu cortex-m4
.thumb

#define A #5
#define B #7
#define C #3

// Global memory locations.
.global vtable_kernel
.global __kernel_reset__

.type vtable_kernel, %object
.type __kernel_reset__, %function

.section .interrupt_vector

vtable_kernel:
	.word __stack_start 
	.word __kernel_reset__+1
	.size vtable_kernel, .-vtable_kernel

.section .rodata
	data: .asciz "kernel started!\n"
	final: .asciz "Value in register #3: "

.section .text
__kernel_reset__:
	ldr r0, =data
	bl dbgput_line
    
// ((a & b) >> 1) + c!
	mov r0, A
	and r0, B        // A & B
	lsr r1, r0, #1   // (A & B) >> 1
	mov r0, #1
	mov r2, C
	bl factorial
	add r3, r0, r1   // ((A & B) >> 1) + C!
        
	ldr r0, =final
	bl dbgput
	mov r0, r3
	bl dbgput_num
    
end:
	b end

factorial:
	push { lr }
	.fact_calc:
		mul r0, r2
		subs r2, #1
		bne .fact_calc
	pop { pc }

Using the .section .interrupt_vector directives, we specify a vector interrupt table. The first row of the table defines the initial state of the Stack Pointer, ie the address from which the stack begins. Next, we specify the __kernel_reset__ program address to handle the RESET exception, indicating the use of thumb instructions.

This program outputs the data string to the debug console, then executes an expression from the previous lab, outputs the contents of r3 to the debug console and is in an infinite loop. To display the result of the program, the procedure dbgput_num is used, the principle of operation can be found in print.s. To output the contents of a certain register, you need to copy its value to r0 and only then call the procedure.

Now consider an example of the bootloader:

Content of bootloader.S

.syntax unified
.cpu cortex-m4
//.fpu softvfp
.thumb

.global bootload

.section .rodata
	image: .incbin "kernel.bin"
	end_of_image:
	str_boot_start: .asciz "bootloader started"
	str_boot_end: .asciz "bootloader end"
	str_boot_indicate: .asciz "#"


.section .text

bootload:
	ldr r0, =str_boot_start
	bl dbgput_line
	ldr r0, =end_of_image
	ldr r1, =image
	ldr r2, =_ram_start

	sub r6, r0, r1
	add r4, r6, r2

loop:
        ldr r3, [r0], #-4
        str r3, [r4], #-4
        cmp r0, r1
        bhi loop

bl newline
ldr r0, =str_boot_end
bl dbgput_line

ldr lr, =bootload_end
add lr, #1
ldr r2, =_ram_start

add r2, #4 // go to __reset_kernel__
ldr r0, [r2]
bx r0


bootload_end:
	b bootload_end

The beginning of the file is the same as in kernel.S. Next, we specify the label that will be responsible for loading the bootload.S into RAM as global, so that this label is visible from start.S. Next, in the .rodata section, we create strings of ASCII characters to verify that the program is working properly, and image, end_of_image labels that contain the memory address where the program begins and ends (end_of_image points to the next word after the last word of the program).

For example, we will load the program sequentially, using the instructions ldr and str:

ldr r0, =str_boot_start
	bl dbgput_line
	ldr r0, =end_of_image
	ldr r1, =image
	ldr r2, =_ram_start

You must first load the start and end addresses of the program and the start address of the RAM into the registers. Also using the dbgput_line procedure, the string str_boot_start is displayed in the debug console. You can get acquainted with the principle of operation of procedures in the file print.S.

After loading the start and end addresses of the program into the appropriate registers, we will load it sequentially into the RAM:

sub r6, r0, r1
add r4, r6, r2

loop:
        ldr r3, [r0], #-4
        str r3, [r4], #-4
        cmp r0, r1
        bhi loop

First, using the ldr statement, we load the word of the program located at the address in r0, then unload it into RAM at the address in r4, and move on to the next word. The cycle ends as soon as the last word of the program is loaded.

Once the load is complete, all you have to do is go to the beginning of RAM to start running the loaded program:

bl newline
ldr r0, =str_boot_end
bl dbgput_line

Using the newline and dbgput_line procedures, the text is output to the debug console, after which we load the start address of the RAM with the ldr command:

ldr lr, =bootload_end
add lr, #1
ldr r2, =_ram_start

Since there is a vector table at the beginning of the program, we need to go to the next word that contains the address of the subroutine that is responsible for handling the RESET exception. In this case, this is the address of the __reset_lernel__ subroutine:

add r2, #4 // go to __reset_kernel__
ldr r0, [r2]
bx r0

bootload_end:
	b bootload_end

🚀 How To Run

Build the project with make:

>>> make

Start the qemu emulator with make qemu:

>>> make qemu

In another terminal, start the gdb debugger with the command gdb-multiarch firmware.elf. And run the program step by step. Demonstrate the value of the registers.

(gdb) target extended-remote:1234
(gdb) layout regs
(gdb) step

Alternatively, once you’ve connected to the chip, you type continue, wait a few seconds, and then hit Ctrl+C. If it asks, ‘Give up waiting?’, enter y for ‘yes’. After the program has run for a bit and then stopped, you can enter the info registers or layout regs command

(gdb) target extended-remote:1234
(gdb) continue
(gdb) layout regs