OSDev.org

I am begin programming the 'task manager' of my OS.

By now, I like to switch from the kernel to another task, both at ring 0.

My kernel is loaded at 0x50000 but the GDT and TSS(s) are outside the kernel. The GDT is at 0x3800 and the TSS(s) area is at 0x35800. I must use code to put data at these structures!

The OS define:

; Linear adresses
dir_GDT equ 0x3800         ; GDT
dir_LDT_ini equ 0x34800    ; LDT(s) for the tasks
dir_TSS_ini equ 0x35800    ; TSS(s) for the tasks
dir_KERNEL equ 0x50000     ; Kernel linear address
...

; Descriptors
desc_lineal equ 8          ; Lineal descriptor
desc_kernel_c equ 16       ; code's kernel descriptor
desc_kernel_d equ 24       ; data's kernel descriptor
desc_LDT_ini equ 32        ; 128 LDT descriptors (not used)
desc_TSS_ini equ 1056      ; 128 TSS descriptors
...

I use this code to load the TSS descriptors for the kernel and task_1,
'ebx' point to the first 'TSS descriptor'(ring 0), and 'gs' to the lineal descriptor:

; Kernel descriptor

mov eax, DWORD dir_TSS_ini    ; First TSS
mov [gs:0 + ebx], WORD 0x67   ; Limit 0-15
mov [gs:2 + ebx], ax          ; Base 0-15
shr eax, 16
mov [gs:4 + ebx], al          ; Base 16-23
mov [gs:5 + ebx], BYTE 0x89   ; P=1, DPL=0, TSS32
mov [gs:6 + ebx], BYTE 0x00   ; G=0
mov [gs:7 + ebx], ah          ; Base 24-31

add ebx, 8                    ; Next descriptor

; task_1 descriptor

mov eax, DWORD dir_TSS_ini + 104  ; Second TSS

mov [gs:0 + ebx], WORD 0x67  ; Limit 0-15
mov [gs:2 + ebx], ax         ; Base 0-15
shr eax, 16
mov [gs:4 + ebx], al         ; Base 16-23
mov [gs:5 + ebx], BYTE 0x89  ; P=1, DPL=0, TSS32
mov [gs:6 + ebx], BYTE 0x00  ; G=0
mov [gs:7 + ebx], ah         ; Base 24-31

The OS switchs to protected mode... Then fill the kernel's TSS with the LTR instruction:

mov ax, desc_TSS_ini         ; first TSS descriptor
ltr ax

Then the OS fill the task_1's TSS with this code. 'gs' point to the lineal descriptor:

mov ebx, DWORD dir_TSS_ini   ; first TSS
lea eax, [tarea_usuario] ; Entry point

; gs:ebx+104 points to the task_1' TSS. It is right?

mov [gs:ebx + 104 + 0x00], DWORD 0
mov [gs:ebx + 104 + 0x04], DWORD 0
mov [gs:ebx + 104 + 0x08], DWORD 0
mov [gs:ebx + 104 + 0x0C], DWORD 0
mov [gs:ebx + 104 + 0x10], DWORD 0
mov [gs:ebx + 104 + 0x14], DWORD 0
mov [gs:ebx + 104 + 0x18], DWORD 0
mov [gs:ebx + 104 + 0x1C], DWORD 0
mov [gs:ebx + 104 + 0x20], eax                   ; 'eip'
mov [gs:ebx + 104 + 0x24], DWORD 0
mov [gs:ebx + 104 + 0x28], DWORD 0
mov [gs:ebx + 104 + 0x2C], DWORD 0
mov [gs:ebx + 104 + 0x30], DWORD 0
mov [gs:ebx + 104 + 0x34], DWORD 0
mov [gs:ebx + 104 + 0x38], esp                   ; 'esp' (?)
mov [gs:ebx + 104 + 0x3C], DWORD 0
mov [gs:ebx + 104 + 0x40], DWORD 0
mov [gs:ebx + 104 + 0x44], DWORD 0
mov [gs:ebx + 104 + 0x48], WORD desc_kernel_d    ; 'es'
mov [gs:ebx + 104 + 0x4C], WORD desc_kernel_c    ; 'cs'
mov [gs:ebx + 104 + 0x50], WORD desc_kernel_d    ; 'ss'
mov [gs:ebx + 104 + 0x54], WORD desc_kernel_d    ; 'ds'
mov [gs:ebx + 104 + 0x58], WORD desc_kernel_d    ; 'fs'
mov [gs:ebx + 104 + 0x5C], WORD desc_kernel_d    ; 'gs'
mov [gs:ebx + 104 + 0x60], DWORD 0
mov [gs:ebx + 104 + 0x64], DWORD 0

Finally, the OS switch to task_1 with the code:

call desc_TSS_ini+8:0

This seems to run well, but with a second call the computer reset.

Somebody know where my error is...

Thank you very much!

pepito

"This seems to run well, but with a !!!second!!! call the computer reset."
Where is the second call being made? Could you give the src of the task(s) code.
Can't you first try to do multitasking between normal task(not part of the kernel). For instance both tasks can switch some char on the screen at different locations.
Anton.

> Where is the second call being made?

Immediately, I do two calls:

call desc_TSS_ini+8:0
call desc_TSS_ini+8:0

> Could you give the src of the task(s) code?

Yes, the code just display a message on the screen:

tarea_usuario:

mov esi, some_message
call message

iret

Thank you!

pepito

Well, I don't know if this is your current problem, but if I recall correctly, when you call another task, your current task is marked busy.  A task marked busy cannot be called again untill the busy mark is removed.  To return to the calling task, you must do a far return to it.  Alternatively, you can jmp into the other task instead of calling it, which clears the busy flag before switching tasks.

Look this up before you take my word for it.  I don't use the TSS for multitasking in my kernel, but this is my recollection from when I read the documentation.

Why not use simple stack switching for your first attempt at multitasking.
In fact, I find it much easier to control than TSS. In your case, it would probably also be faster to simply change ss and esp (or just esp).