xOS is a simple shell(no file support yet ) with a few neat commands.

me239 wrote:

xOS is a simple shell(no file support yet ) with a few neat commands.

Good job.
I took the liberty of translating xOS to 32 bit protected mode to show how easy it is to "overnight" move up to PM32,
(which will also boot and run on any PC that can boot from USB flash) with the same ease and simplicity of 16 bit real mode.
The PM32 boot sector included with the aeBIOS distribution loads a PM32 flat binary file called kernel.bin to 100000h.
This boot sector (located at LBA 36) can be changed or modified. The cmd.bin file (without the boot loader) was renamed
"kernel.bin" and copied to the root directory of the aeBIOS image after it is transferred to the USB flash drive.

WOW, thanks. I would really like to give it file support. Can you help me accomplish this?

me239 wrote:

WOW, thanks. I would really like to give it file support. Can you help me accomplish this?

If your talking realmode a good starting point would be bootprog
http://alexfru.chat.ru/epm.html#bootprog

I have converted it to fasm, let me know if you want a copy.

WOW, thanks. I would really like to give it file support. Can you help me accomplish this?

me239 wrote:

WOW, thanks. I would really like to give it file support. Can you help me accomplish this?

Take a look at the source code for the PM32 boot sector / boot loader. This is the boot sector (located at LBA 36)
which aeBIOS loads to 7C00h.
This code can convert and look up a file name that the user enters on the command line. If found in the root directory
it can be loaded to say 200000h (2 MEG) and run. These apps would have "org 200000h" and could return to the shell
with "ret".

I've put aside the FAT12 until the weekend and instead I'm simulating int 21h DOS commands using the IVT!

pm_idtr:
  dw NUMBER_OF INTERRUPT_HANDLERS*8-1
  dd POINTER_TO_IDT    

PM_IDTR                           equ 520h
  mov esi, [PM_IDTR+2]          ; esi points to IDT    

  jmp DWORD [saved_interrupt]    

PM_IDTR                           equ 520h
use32
org 100000h
  mov esi, [PM_IDTR+2]          ; esi points to IDT
  
; save the original handler - not required for int 21h
  mov eax, [esi+21h*8]
  rol eax, 16
  mov ax, [esi+21h*8+6]
  rol eax, 16
  mov [saved_int_21], eax
; option not required
  
  mov WORD [esi+21h*8], int_21 AND 0FFFFh
  mov WORD [esi+21h*8+6], int_21 SHR 16

  mov esi, message
  call print
  xor eax, eax
  int 16h
  int 19h

print:
  mov ah, 2                     ; Write Character To Standard Output
.1:
  mov dl, [esi]
  lea esi, [esi+1]
  test dl, dl
  je .2
  int 21h
  jmp .1
.2:
  ret

message:
  db 'Using int 21h is fun!', 0

int_21:
  pusha
  cmp ah, 2
  jne .1
  mov ah, 0Eh
  xor bh, bh
  mov al, dl
  int 10h
.1:
  popa
; optionally can be simply replace with "iret"
  jmp DWORD [saved_int_21]
saved_int_21: dd 0
; option      

me239 wrote:

I've put aside the FAT12 until the weekend and instead I'm simulating int 21h DOS commands using the IVT!

Setting up an interrupt handler in PM32 with the IDT is only slightly different.
The format of the Protected Mode Interrupt Descriptor Table Pointer is:

Code:

pm_idtr:
  dw NUMBER_OF INTERRUPT_HANDLERS*8-1
  dd POINTER_TO_IDT    

With aeBIOS the PM_IDTR is always located in a global variable at 520h. So to access the IDT:

Code:

PM_IDTR                           equ 520h
  mov esi, [PM_IDTR+2]          ; esi points to IDT    

Each IDT entry is 8 bytes long and contains protection, type and segment information which in a single segment
unprotected memory model can be ignored. The flat 32 bit address of the handler is stored in the entry such that the
lower 16 bits is in the first 2 bytes and the upper 16 bits is in the last 2 bytes.
Hooking a handled interrupt (aeBIOS provides default handlers for all interrupts - most of which merely do an "iret"),
involves saving and replacing the handler address. When the interrupt occurs the new handler will be executed, the last
instruction should be:

Code:

  jmp DWORD [saved_interrupt]    

to return to the original handler which when finished will return to the interrupted code.
Here is a simple framework example of establishing a new handler.

Code:

PM_IDTR                           equ 520h
use32
org 100000h
  mov esi, [PM_IDTR+2]          ; esi points to IDT
  
; save the original handler - not required for int 21h
  mov eax, [esi+21h*8]
  rol eax, 16
  mov ax, [esi+21h*8+6]
  rol eax, 16
  mov [saved_int_21], eax
; option not required
  
  mov WORD [esi+21h*8], int_21 AND 0FFFFh
  mov WORD [esi+21h*8+6], int_21 SHR 16

  mov esi, message
  call print
  xor eax, eax
  int 16h
  int 19h

print:
  mov ah, 2                     ; Write Character To Standard Output
.1:
  mov dl, [esi]
  lea esi, [esi+1]
  test dl, dl
  je .2
  int 21h
  jmp .1
.2:
  ret

message:
  db 'Using int 21h is fun!', 0

int_21:
  pusha
  cmp ah, 2
  jne .1
  mov ah, 0Eh
  xor bh, bh
  mov al, dl
  int 10h
.1:
  popa
; optionally can be simply replace with "iret"
  jmp DWORD [saved_int_21]
saved_int_21: dd 0
; option      

Assemble as kernel.bin, copy to the root directory and boot.