Switch protcted mode (32bits) to real mode (16bits) in asm, low level

I am creating an OS, and I need to call the BIOS in real mode, I tried this code but it has a bug: The error on boch is

00002172825e[CPU0 ] check_cs: conforming code seg descriptor dpl > cpl

The code is :

    global go16
;______________________________________________________________________________________________________
;Switch to 16-bit real Mode
;IN/OUT:  nothing

go16:
    [BITS 32]

    cli                 ;Clear interrupts
    pop edx             ;save return location in edx
    jmp 0x20:PM16       ;Load CS with selector 0x20

;For go to 16-bit real mode, first we have to go to 16-bit protected mode
    [BITS 16]
PM16:
    mov ax, 0x28        ;0x28 is 16-bit protected mode selector.
    mov ss, ax  
    mov ds, ax
    mov es, ax
    mov gs, ax
    mov fs, ax
    mov sp, 0x7c00+0x200    ;Stack hase base at 0x7c00+0x200    

    
    mov eax, cr0
    and eax, 0xfffffffe ;Clear protected enable bit in cr0
    mov cr0, eax   
    jmp 0x50:realMode   ;Load CS and IP


realMode:
;Load segment registers with 16-bit Values.
    mov ax, 0x50
    mov ds, ax
    mov fs, ax
    mov gs, ax
    mov ax, 0
    mov ss, ax
    mov ax, 0
    mov es, ax
    hlt
    mov sp, 0x7c00+0x200    

    cli
    lidt[.idtR]     ;Load real mode interrupt vector table
    sti

    push 0x50       ;New CS
    push dx         ;New IP (saved in edx)
    retf            ;Load CS, IP and Start real mode

;Real mode interrupt vector table
.idtR: 
    dw 0xffff       ;Limit
    dd 0            ;Base

The program stops at jmp 0x20:PM16
because he don't like 0x20 I need to call the bios to switch graphic mode because I can't do that in my bootloader Thank you for your answer PS : My gdt is :

init_gdt_desc(0x0, 0x0, 0x0, 0x0, &kgdt[0]);
init_gdt_desc(0x0, 0xFFFFF, 0x9B, 0x0D, &kgdt[1]);      
init_gdt_desc(0x0, 0xFFFFF, 0x93, 0x0D, &kgdt[2]);    
init_gdt_desc(0x0, 0x0, 0x97, 0x0D, &kgdt[3]);          

//user segment
init_gdt_desc(0x0, 0xFFFFF, 0xFF, 0x0D, &kgdt[4]); /* ucode */
init_gdt_desc(0x0, 0xFFFFF, 0xF3, 0x0D, &kgdt[5]); /* udata */
init_gdt_desc(0x0, 0x0, 0xF7, 0x0D, &kgdt[6]); /* ustack */

For a jmp or call to a different segment you need to obey one of the following priviledge rules:

• target DPL is equal to CPL
• target DPL is less or equal to CPL, but the target conforming bit is set

Looking at your selectors, the target DPL is 3 and the CPL is 0.

So to use jmp 0x20:PM16, change

init_gdt_desc(0x0, 0xFFFFF, 0xFF, 0x0D, &kgdt[4]); /* ucode */

into

init_gdt_desc(0x0, 0xFFFFF, 0x9F, 0x0D, &kgdt[4]); /* ucode */

;Real mode interrupt vector table
.idtR: 
    dw 0xffff       ;Limit
    dd 0            ;Base

The correct limit would be 0x03FF (1023 in decimal)

[edit]

Since the goal is to go to real mode the BIG bit in the descriptors should be off to select 16-bit, and the GRANULARITY bit should be off to limit memory access to just 1MB.

init_gdt_desc(0x0, 0xFFFF, 0x9E, 0x0, &kgdt[4]); /* ucode */
init_gdt_desc(0x0, 0xFFFF, 0x92, 0x0, &kgdt[5]); /* udata */

The order of things in your current program is different from what you can find here about switching from protected mode to real mode.