I've been debugging this for hours, and still no progress.
I've set up a GDT, and IDT, and within the IDT I have an ISR for the keyboard. I also have set up both PICs. Here's my kernel code:
Code:
MB_FLAGS equ 1<<0 | 1<<1 | 1<<2 ; 0x1: all boot modules aligned on 4KB boundaries, even
; though i don't think i'll need to use this
; 0x10: memory information available
; 0x100: means i can set parameters about the video output
MB_MAGICNUMBER equ 0x1BADB002
MB_CHKSUM equ -(MB_FLAGS + MB_MAGICNUMBER)
MB_MODETYPE equ 1 ; textmode
MB_WIDTH equ 80
MB_HEIGHT equ 24
MB_DEPTH equ 0
PIC1 equ 0x20 ; IO base address for master 8259 PIC
PIC2 equ 0xa0 ; addr for slave PIC
PIC1_CMD equ PIC1
PIC1_DAT equ PIC1 + 1
PIC2_CMD equ PIC2
PIC2_DAT equ PIC2 + 1
ICW1 equ 00010001b ; ICW4 needed
ICW2_MASTER equ 00110000b ; 0x60
ICW2_SLAVE equ 01000000b ; 0x70
ICW3_MASTER equ 00000100b
ICW3_SLAVE equ 00000010b
ICW4 equ 00000001b ; 8086/88 mode
section .multiboot_header:
align 4
multiboot_start:
dd MB_MAGICNUMBER
dd MB_FLAGS
dd MB_CHKSUM
dd 0,0,0,0,0 ; 5 * 4 zero bytes in a row, because i'm not using flag 16
dd MB_MODETYPE
dd MB_WIDTH
dd MB_HEIGHT
dd MB_DEPTH
multiboot_length equ $ - multiboot_start
section .bss
align 16
;; the stack is defined here.
;; defined in .bss to save kernel space, since it'll just say
;; "16384 uninitialised bytes here", rather than actually list
;; that many bytes out in the object file!
stack_bottom:
resb 16384
stack_top:
section .isr ; section for ISRs
section .text ; now the actual kernel entry point is in this section
global _start:function (_start.end - _start) ; make the object file store the length of the _start symbol
; I have two segments, gdt_code and gdt_data
; they're both kernel-only segments, so they can't be accessed from userspace code
; for security reasons.
; this is because i've set them to be only accessible by ring 0 code, which is the
; kernel.
gdt_start:
gdt_null:
dq 0 ; 8 bytes of nothing
gdt_code:
dw 0xffff ; bits 0-15 of the length of the code segment
dw 0x0000 ; bits 0-15 of the base of the code segment
db 0x00 ; bits 16-23 of the base of the code segment
db 0x9a ; the access byte. this says that it can only be accessed by the kernel (ring 0)
db 11001111b ; bits 16-19 of the length of the segment, also flags saying 32 bit prot., and 4k blocks
db 0x00 ; bits 24-31 of the base
gdt_data:
dw 0xffff ; bits 0-15 of length
dw 0x0000 ; bits 0-15 of base
db 0x00 ; bits 16-23 of base
db 0x92 ; access byte. non executable, only accessible by kernel (ring 0)
db 11001111b ; bits 16-19 of length, and also flags
db 0x00 ; bits 24-31 of base
gdt_length equ $ - gdt_start - 1
gdt_descriptor dw gdt_length ; the length of the gdt
dd gdt_start ; the address of the gdt (both to be loaded in from C)
idt_start: ; each idt entry is 8 bytes long
resd 0x60 * 2 ; pad with doublewords to get up to 0x60th entry
dq 0 ; PIT timer
idt_keyboard:
dw 0x0000 ; bits 0-15 of offset
dw 0x0008 ; code segment selector
db 0x00 ; unused
db 10001110b ; type
dw 0x0000 ; bits 16-31 of offset
idt_length equ $ - idt_start
idt_descriptor dw idt_length
dd idt_start
pic_init:
; https://www.eeeguide.com/programming-8259/
cli
; I will map the master PIC to 0x60+0:7 = 0110,0000b
; and the slave PIC to 0x70+0:7 = 0111,0000b (which is actually the default for it anyway)
mov al, ICW1
out PIC1_CMD, al
out PIC2_CMD, al
mov al, ICW2_MASTER
out PIC1_DAT, al
mov al, ICW2_SLAVE
out PIC2_DAT, al
mov al, ICW3_MASTER
out PIC1_DAT, al
mov al, ICW3_SLAVE
out PIC2_DAT, al
mov al, ICW4
out PIC1_DAT, al
out PIC2_DAT, al
xchg bx, bx
mov al, 0xfd ; set masks to 0
out PIC1_DAT, al
mov al, 0xff
out PIC2_DAT, al
;mov al, 0xfd
;out 0x60, al
xchg bx, bx
; Now both of the PICs are initialised correctly
sti
ret
isr_keyboard:
xchg bx, bx
pusha
xchg bx, bx
mov al, 0x20 ; EOI
out PIC1_CMD, al
xchg bx, bx
popa
iret
_start: ; the actual entry point to the kernel!
mov esp, stack_top ; setup the stack, because obviously it's nice to have a stack, and also
; C programs need a stack to run
cli
lgdt [gdt_descriptor]
; put addresses in the IDT
mov eax, isr_keyboard ; eax = 0xUUUULLLL, U=upper L=lower
mov word [idt_keyboard], ax ; put the lower 16 bits into the IDT
shr eax, 16 ; set ax to the upper bits of eax
mov word [idt_keyboard + 6], ax ; put the upper 16 bits into the IDT with an offset
xchg bx, bx
lidt [idt_descriptor]
xchg bx, bx
call pic_init
;xchg bx, bx
.hltloop
jmp .hltloop
.end: ; marks the end of the _start symbol's code
I realise the code is quite long to read through, hopefully the comments will help. So, my problem is that when the kernel reaches the loop at the end, I try to press a key on my keyboard. When I do this, I get a long error log from bochs. Here it is:
Code:
00322545000e[CPU0 ] interrupt(): gate descriptor is not valid sys seg (vector=0x31)
00322545000e[CPU0 ] interrupt(): gate descriptor is not valid sys seg (vector=0x0d)
00322545000e[CPU0 ] interrupt(): gate descriptor is not valid sys seg (vector=0x08)
00322545000i[CPU0 ] CPU is in protected mode (active)
00322545000i[CPU0 ] CS.mode = 32 bit
00322545000i[CPU0 ] SS.mode = 32 bit
00322545000i[CPU0 ] EFER = 0x00000000
00322545000i[CPU0 ] | EAX=000000ff EBX=00010000 ECX=00000000 EDX=00000000
00322545000i[CPU0 ] | ESP=00105000 EBP=00000000 ESI=00000000 EDI=00000000
00322545000i[CPU0 ] | IOPL=0 id vip vif ac vm RF nt of df IF tf sf zf af pf cf
00322545000i[CPU0 ] | SEG sltr(index|ti|rpl) base limit G D
00322545000i[CPU0 ] | CS:0010( 0002| 0| 0) 00000000 ffffffff 1 1
00322545000i[CPU0 ] | DS:0018( 0003| 0| 0) 00000000 ffffffff 1 1
00322545000i[CPU0 ] | SS:0018( 0003| 0| 0) 00000000 ffffffff 1 1
00322545000i[CPU0 ] | ES:0018( 0003| 0| 0) 00000000 ffffffff 1 1
00322545000i[CPU0 ] | FS:0018( 0003| 0| 0) 00000000 ffffffff 1 1
00322545000i[CPU0 ] | GS:0018( 0003| 0| 0) 00000000 ffffffff 1 1
00322545000i[CPU0 ] | EIP=001003a4 (001003a4)
00322545000i[CPU0 ] | CR0=0x60000011 CR2=0x00000000
00322545000i[CPU0 ] | CR3=0x00000000 CR4=0x00000000
00322545000e[CPU0 ] exception(): 3rd (13) exception with no resolution, shutdown status is 00h, resetting
00322545000i[SYS ] bx_pc_system_c::Reset(HARDWARE) called
00322545000i[CPU0 ] cpu hardware reset
...
(There is more after this error message, however it's just details of the emulator doing a CPU restart, so not relevant I don't think.)
My first thought was that I might have loaded the address of the keyboard ISR into the IDT incorrectly, but I checked the IDT in the bochs debugger and followed the address which the IDT had for 0x61, and it was the correct piece of code. So that's working.
So then I took a closer look at the interrupt() gate descriptor... errors, and can't work out why the interrupt vector 0x31 would ever be called. I don't think it should be called, because:
It's not one of the exception interrupt vectors
It's not being called from either of the PICs, since they're remapped 0x60 and 0x70
So... any ideas?
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