OSDev.org

Code: Select all

; This macro creates a 'stub' for an ISR that does not
; push its own error code, a dummy error code (0) is used instead.
; NOTE: The general handler expects the interrupt number
; and error code on the stack.
%macro ISR_STUB_NOERRORCODE 1
	[GLOBAL isr%1]
	isr%1:
		cli					; disable interrupts
		push 0				; push a dummy error code
		push dword %1		; push the interrupt number
		jmp common_stub_isr	; use the common ISR handler to handle this ISR
%endmacro

; This macro creates a 'stub' for an ISR that pushes its own error code.
; NOTE: The general handler expects the interrupt number
; and error code on the stack.
%macro ISR_STUB 1
	[GLOBAL isr%1]
	isr%1:
		cli					; disable interrupts
		push dword %1		; push the interrupt number
		jmp common_stub_isr	; use the common ISR handler to handle this ISR
%endmacro

...

; set up the ISR/IRQ stubs
ISR_STUB_NOERRORCODE	0
ISR_STUB_NOERRORCODE	1

...

; a C function that writes a string to the console
[EXTERN debugconsole_writeline]

fmt: db 'AD', 0  ; Acronym for 'Assembly-Done', I print this out for debugging...

; Common ISR stub that saves all needed information to the stack then calls
; the C handler.
common_stub_isr:
	pusha                    ; pushes edi, esi, ebp, esp, ebx, edx, ecx, eax
	
	xor eax, eax
	mov ax, ds               ; save the data segment descriptor
	push eax                 ; ...

	; The kernel data segment is the 3rd gdt entry (index 2),
	; each of which is 8 bytes.  Thus its offfset is (2 * 8) = 16
	; which is 0x10 in hex.
	;
	; We need to update all of the segment registers to reflect this.
	; 
	; For now, these registers are always 0x10 (this is setup when
	; the GDT is created, and hasn't been touched since; which
	; technically makes this code redundant).
	mov ax, 0x10
	mov ds, ax
	mov es, ax
	mov fs, ax
	mov gs, ax
	
	call IsrHandler	; call C code
	
	; print a notification to the screen saying we got this far
	; NOTE: This does result in 'AD' being printed to the screen.
	push fmt
	call debugconsole_writeline
	add esp, 4
	
	pop eax		; reload the original segment descriptors
	mov ds, ax
	mov es, ax
	mov fs, ax
	mov gs, ax
	
	; print a notification to the screen saying we got this far
	; Starting here, AD is no longer printed (which is why
	; I have come to believe the code above is the culprit)
	push fmt
	call debugconsole_writeline
	add esp, 4
	
	popa		; pops edi, esi, ebp, esp, eb, edx, ec, eax
	add esp, 8	; cleans up the pushed error code and ISR number
	
	; print a notification to the screen saying we got this far
	push fmt
	call debugconsole_writeline
	add esp, 4
	
	sti		; enable interrupts
	
	iret		; pops 5 things at once: CS, EIP, EFLAGS, SS, and ESP

Code: Select all

	pop eax		; reload the original data segment descriptor
	mov ds, ax
	mov es, ax
	mov fs, ax
	mov gs, ax

Code: Select all

...

call IsrHandler   ; call C code 

...

pop eax      ; reload the original segment descriptors   <-- value of EAX after this is 0x10
mov ds, ax  <-- GPF
mov es, ax
mov fs, ax
mov gs, ax 

Code: Select all

; The kernel data segment is the 3rd gdt entry (index 2),
; each of which is 8 bytes.  Thus its offfset is (2 * 8) = 16
; which is 0x10 in hex.
;
; We need to update all of the segment registers to reflect this.
;
; For now, these registers are always 0x10 (this is setup when
; the GDT is created, and hasn't been touched since; which
; technically makes this code redundant).
mov ax, 0x10
mov ds, ax
mov es, ax
mov fs, ax
mov gs, ax