How to increase memory for kernel variables and functions in C?

Question

assembler

When reaching or more than a certain number of variables or functions, all string literals in the C core stop working, how can I increase the memory for them?

mbr.asm loader:

[org 0x7c00]
KERNEL_OFFSET equ 0x1000 ; The same one we used when linking the kernel

mov [BOOT_DRIVE], dl ; Remember that the BIOS sets us the boot drive in 'dl' on boot
mov bp, 0x8000 ; 0x8000
mov sp, bp

mov bx, MSG_16BIT_MODE
call print16
call print16_nl

call load_kernel ; read the kernel from disk
call switch_to_32bit ; disable interrupts, load GDT,  etc. Finally jumps to 'BEGIN_PM'
jmp $ ; Never executed

%include "boot/print-16bit.asm"
%include "boot/print-32bit.asm"
%include "boot/disk.asm"
%include "boot/gdt.asm"
%include "boot/switch-to-32bit.asm"

[bits 16]
load_kernel:
    mov bx, MSG_LOAD_KERNEL
    call print16
    call print16_nl

    mov bx, KERNEL_OFFSET ; Read from disk and store in 0x1000
    mov dh, 54 ;31 or 54
    mov dl, [BOOT_DRIVE]
    call disk_load
    ret

[bits 32]
BEGIN_32BIT:
    mov ebx, MSG_32BIT_MODE
    call print32
    call KERNEL_OFFSET ; Give control to the kernel
    jmp $ ; Stay here when the kernel returns control to us (if ever)


BOOT_DRIVE db 0 ; It is a good idea to store it in memory because 'dl' may get overwritten
MSG_16BIT_MODE db "Started in 16-bit Real Mode", 0
MSG_32BIT_MODE db "Landed in 32-bit Protected Mode", 0
MSG_LOAD_KERNEL db "Loading kernel into memory", 0

; padding
times 510 - ($-$$) db 0
dw 0xaa55

Boot from disk.asm:

; load 'dh' sectors from drive 'dl' into ES:BX
disk_load:
    pusha
    ; reading from disk requires setting specific values in all registers
    ; so we will overwrite our input parameters from 'dx'. Let's save it
    ; to the stack for later use.
    push dx

    mov ah, 0x02 ; ah <- int 0x13 function. 0x02 = 'read'
    mov al, dh   ; al <- number of sectors to read (0x01 .. 0x80)
    mov cl, 0x02 ; cl <- sector (0x01 .. 0x11)
                 ; 0x01 is our boot sector, 0x02 is the first 'available' sector
    mov ch, 0x00 ; ch <- cylinder (0x0 .. 0x3FF, upper 2 bits in 'cl')
    ; dl <- drive number. Our caller sets it as a parameter and gets it from BIOS
    ; (0 = floppy, 1 = floppy2, 0x80 = hdd, 0x81 = hdd2)
    mov dh, 0x00 ; dh <- head number (0x0 .. 0xF)

    ; [es:bx] <- pointer to buffer where the data will be stored
    ; caller sets it up for us, and it is actually the standard location for int 13h
    int 0x13      ; BIOS interrupt
    jc disk_error ; if error (stored in the carry bit)

    pop dx
    cmp al, dh    ; BIOS also sets 'al' to the # of sectors read. Compare it.
    jne sectors_error
    popa
    ret


disk_error:
    mov bx, DISK_ERROR
    call print16
    call print16_nl
    mov dh, ah ; ah = error code, dl = disk drive that dropped the error
    call print16_hex ; check out the code at http://stanislavs.org/helppc/int_13-1.html
    jmp disk_loop

sectors_error:
    mov bx, SECTORS_ERROR
    call print16

disk_loop:
    jmp $

DISK_ERROR: db "Disk read error", 0
SECTORS_ERROR: db "Incorrect number of sectors read", 0

gdt.asm:

gdt_start: ; don't remove the labels, they're needed to compute sizes and jumps
    ; the GDT starts with a null 8-byte
    dd 0x0 ; 4 byte
    dd 0x0 ; 4 byte

; GDT for code segment. base = 0x00000000, length = 0xfffff
; for flags, refer to os-dev.pdf document, page 36
gdt_code:
    dw 0xffff    ; segment length, bits 0-15
    dw 0x0       ; segment base, bits 0-15
    db 0x0       ; segment base, bits 16-23
    db 10011010b ; flags (8 bits)
    db 11001111b ; flags (4 bits) + segment length, bits 16-19
    db 0x0       ; segment base, bits 24-31

; GDT for data segment. base and length identical to code segment
; some flags changed, again, refer to os-dev.pdf
gdt_data:
    dw 0xffff
    dw 0x0
    db 0x0
    db 10010010b
    db 11001111b
    db 0x0

gdt_end:

; GDT descriptor
gdt_descriptor:
    dw gdt_end - gdt_start - 1 ; size (16 bit), always one less of its true size
    dd gdt_start ; address (32 bit)

; define some constants for later use
CODE_SEG equ gdt_code - gdt_start
DATA_SEG equ gdt_data - gdt_start

code loader in C - kernel-entry.asm:

global _start
[bits 32]

_start:
    [extern _start_kernel] ; Define calling point. Must have same name as kernel.c 'main' function
    call _start_kernel ; Calls the C function. The linker will know where it is placed in memory
    jmp $

GNU Linker linker.ld:

ENTRY(_start)

SECTIONS
{
  . = 0x1000;
  .text : { *(.text) }
  .data : { *(.data) }
  .bss  : { *(.bss) *(COMMON) }
}

compilation on Windows compile.bat:

gcc -ffreestanding -m32 -g -c kernel/kernel.c -o kernel/kernel.o -std=c11
gcc -ffreestanding -m32 -g -c kernel/util.c -o kernel/util.o -std=c11
gcc -ffreestanding -m32 -g -c kernel/mem.c -o kernel/mem.o -std=c11
gcc -ffreestanding -m32 -g -c kernel/system.c -o kernel/system.o -std=c11

gcc -ffreestanding -m32 -g -c drivers/ports.c -o drivers/ports.o -std=c11
gcc -ffreestanding -m32 -g -c drivers/display.c -o drivers/display.o -std=c11
gcc -ffreestanding -m32 -g -c drivers/keyboard.c -o drivers/keyboard.o -std=c11

gcc -ffreestanding -m32 -g -c cpu/idt.c -o cpu/idt.o -std=c11
gcc -ffreestanding -m32 -g -c cpu/isr.c -o cpu/isr.o -std=c11
gcc -ffreestanding -m32 -g -c cpu/timer.c -o cpu/timer.o -std=c11

nasm -fbin boot/mbr.asm -o boot/mbr.bin
nasm -felf boot/kernel-entry.asm -o boot/kernel-entry.o
nasm -felf cpu/interrupt.asm -o cpu/interrupt.o

ld -mi386pe -no-PIE -o kernel.out -Tlinker.ld boot/kernel-entry.o kernel/kernel.o  kernel/util.o kernel/mem.o drivers/ports.o drivers/display.o drivers/keyboard.o cpu/idt.o cpu/interrupt.o cpu/isr.o cpu/timer.o kernel/system.o
objcopy -I elf32-i386 kernel.out kernel.elf
objcopy -O binary kernel.elf kernel.bin
type boot\\mbr.bin kernel.bin > os-image.bin
qemu-system-i386 -fda os-image.bin

readelf -a kernel.elf in two cases: https://pastebin.com/kTgGc2Yd

Answer 1

When reaching or exceeding a certain number of variables or functions, all string literals in the C core stop working

Certain - what is it?
Right here

mov bx, KERNEL_OFFSET ; Read from disk and store in 0x1000
    mov dh, 54 ;31 or 54

You load 54 sectors of the kernel into memory. Is there any correlation?

How can I increase the memory for them?

you did not bring the kernel code, what breaks there, you can only guess, well, you understand.