Re-wrote interrupt handling. Now, flush_idt only initializes IDT entries

32-255. A separate routine in the new file traps.s initializes the first
32 entries with addresses pointing to exception handlers within said
file.

Modified the register_isr function to now accept a descriptor privilege
level which it will assign to the modified IDT entry.

Added a task state segment and corresponding entry to the GDT. The TSS
will store the kernel's stack pointer and stack segment when switching
to userspace. NOTE: The stack pointer MUST be saved manually before
switching to userspace!

Added the framework for a system call interface at interrupt vector 0x80
(128).

8 files changed, 320 insertions, 51 deletions

diff --git a/kernel/boot.s b/kernel/boot.s
index 024abf0..bc42a5d 100644
--- a/kernel/boot.s
+++ b/kernel/boot.s
@@ -1,7 +1,12 @@
+global gdt
+global idt
 global kboot
 global register_isr
+global tss
 extern kmain
+extern syscall_init
 extern timer_init
+extern traps_init
 
 ; Multiboot header
 section .mboothdr
@@ -15,15 +20,32 @@ section .text
 
 kboot:
   cli
+  ; put the stack pointer near the end of conventional memory
+  mov esp, 0x80000
+  ; setup descriptor tables
   call flush_gdt
   call flush_idt
-  ;call timer_init
+  ; last minute setup, then transfer to kmain
+  call timer_init
   call kmain
+  ; if kmain decides to return (which it shouldn't),
+  ; disable interrupts and halt the system.
   cli
   hlt
 
 tss:
-  times 104 db 0
+  dd 0
+  dd 0          ; kernel stack pointer
+  dd 0x10       ; kernel stack segment
+  times 60 db 0
+  dd 0x13       ; ES segment
+  dd 0x0B       ; CS segment
+  dd 0x13       ; SS segment
+  dd 0x13       ; DS segment
+  dd 0x13       ; FS segment
+  dd 0x13       ; GS segment
+  times 8 db 0
+.end:
 
 gdt:
   ; null descriptor
@@ -57,14 +79,31 @@ gdt:
   db 0xCF
   db 0x00
   ; task state segment
+.tss:
+  dq 0  ; we initialize this later
+.end:
 
 gdtp:
-  dw ($-gdt-1)
+  dw (gdt.end-gdt-1)
   dd gdt
 
 flush_gdt:
   push ebp
   mov ebp, esp
+  ; initialize the TSS entry
+  mov eax, tss
+  mov [gdt.tss+2], ax
+  shr eax, 16
+  mov [gdt.tss+4], al
+  mov [gdt.tss+7], ah
+  mov eax, (tss.end-tss-1)
+  mov [gdt.tss], ax
+  shr eax, 16
+  and al, 0x0F
+  mov [gdt.tss+6], al
+  mov al, 0xE9
+  mov [gdt.tss+5], al
+  ; now, load the GDT
   lgdt [gdtp]
   mov ax, 0x10
   mov ds, ax
@@ -74,65 +113,72 @@ flush_gdt:
   mov ss, ax
   jmp 0x08:.end
 .end:
+  ; initialize the task state register
+  mov ax, 0x002B
+  ltr ax
   pop ebp
   ret
 
 idt:
   times (256*8) db 0
+.end:
 
 idtp:
-  dw ($-idt-1)
+  dw (idt.end-idt-1)
   dd idt
 
 int_handler:
   iret
 
-irq_handler:
-  iret
-
 flush_idt:
   push ebp
   mov ebp, esp
+  ; install the exception handlers (vectors 0-31)
+  call traps_init
+  ; install the default handlers (vectors 32-255)
   mov ecx, 0
-  mov edx, idt
+  lea edx, [idt+(32*8)]
 .loop:
   mov eax, int_handler
-  cmp ecx, 0x20
-  jl .skip
-  cmp ecx, 0x2f
-  jg .skip
-  mov eax, irq_handler
-.skip:
   mov [edx], ax
-  mov word [edx+2], 0x08
-  mov byte [edx+4], 0
-  mov byte [edx+5], 0x8E
   shr eax, 16
   mov [edx+6], ax
-  add edx, 8
+  mov word [edx+2], 0x08
+  mov byte [edx+4], 0x00
+  mov byte [edx+5], 0x8E
   inc ecx
-  cmp ecx, 256
+  cmp ecx, (256-32)
   je .end
+  add edx, 8
   jmp .loop
 .end:
+  ; install the system call handler
+  call syscall_init
+  ; load the IDT, initialize the PIC's, and enable interrupts
   lidt [idtp]
   call pic_init
   sti
   pop ebp
   ret
 
-; void register_isr(uint8_t n, void *handler);
+; void register_isr(uint8_t n, uint8_t dpl, void *handler);
 register_isr:
   push ebp
   mov ebp, esp
   mov edx, [ebp+8]
   shl edx, 3
   add edx, idt
-  mov eax, [ebp+12]
+  mov eax, [ebp+16]
   mov [edx], ax
-  add edx, 6
   shr eax, 16
-  mov [edx], ax
+  mov [edx+6], ax
+  mov eax, [ebp+12]
+  and al, 3
+  shl al, 5
+  mov cl, [edx+5]
+  and cl, 0x9F
+  or cl, al
+  mov [edx+5], cl
   pop ebp
   ret
 
diff --git a/kernel/kmain.c b/kernel/kmain.c
index 81fe741..bf6b9ae 100644
--- a/kernel/kmain.c
+++ b/kernel/kmain.c
@@ -10,8 +10,8 @@ extern void userspace_init(void);
 void kmain(void) {
   con_init();
 
-  printf("Kernel booting...\n\n");
-  printf("\nKernel booted!\n");
+  printf("Kernel booting...\n");
+  printf("Kernel booted!\n\n");
 
   userspace_init();
 }
diff --git a/kernel/syscall.c b/kernel/syscall.c
new file mode 100644
index 0000000..8fa5da0
--- /dev/null
+++ b/kernel/syscall.c
@@ -0,0 +1,12 @@
+#include <kernel/con.h>
+#include <stdarg.h>
+#include <stdint.h>
+
+/* main syscall handler */
+void syscall(va_list ap) {
+  uint8_t call;
+  char *s;
+  
+  printf("Call Number: 0x%02x\n", va_arg(ap, uint8_t));
+  printf(va_arg(ap, char*));
+}
diff --git a/kernel/timer.s b/kernel/timer.s
index de01ea6..53c81ed 100644
--- a/kernel/timer.s
+++ b/kernel/timer.s
@@ -17,9 +17,10 @@ timer_init:
   mov ebp, esp
   ; register the tick_handler
   push tick_handler
+  push dword 0
   push dword 0x20
   call register_isr
-  add esp, 8
+  add esp, 12
   ; initialize the pit
   mov ax, 0x36
   out 0x43, al
diff --git a/kernel/traps.s b/kernel/traps.s
new file mode 100644
index 0000000..7b7446b
--- /dev/null
+++ b/kernel/traps.s
@@ -0,0 +1,227 @@
+;
+; traps.s provides entry points to the exception handlers
+; as well as the system call interface.
+;
+
+global syscall_init
+global traps_init
+extern idt
+extern register_isr
+extern syscall
+
+%macro SAVE 0
+  pusha
+  ; save the data segment selectors
+  mov ax, ds
+  push ax
+  ; switch to kernel data segments
+  mov ax, 0x10
+  mov ds, ax
+  mov es, ax
+  mov fs, ax
+  mov gs, ax
+%endmacro
+
+%macro SAVE_ERR 0
+  pusha
+  ; fetch the error code that was pushed
+  mov ebx, [esp+32]
+  ; save the data segment selectors
+  mov ax, ds
+  push ax
+  ; switch to kernel data segments
+  mov ax, 0x10
+  mov ds, ax
+  mov es, ax
+  mov fs, ax
+  mov gs, ax
+%endmacro
+
+%macro RESTORE 0
+  ; restore the data segment selectors
+  pop ax
+  mov ds, ax
+  mov es, ax
+  mov fs, ax
+  mov gs, ax
+  popa
+  iret
+%endmacro
+
+%macro RESTORE_ERR 0
+  ; restore the data segment selectors
+  pop ax
+  mov ds, ax
+  mov es, ax
+  mov fs, ax
+  mov gs, ax
+  popa
+  add esp, 4
+  iret
+%endmacro
+
+traps:
+  dd exc_div
+  dd exc_debug
+  dd exc_nmi
+  dd exc_brk
+  dd exc_ovf
+  dd exc_bounds
+  dd exc_invop
+  dd exc_nomath
+  dd exc_dbf
+  dd exc_coseg
+  dd exc_invtss
+  dd exc_segnotpres
+  dd exc_stackf
+  dd exc_gprot
+  dd exc_pagef
+  dd exc_reserved
+  dd exc_mathf
+  dd exc_alignchk
+  dd exc_machinechk
+  dd exc_simdfp
+  dd exc_virt
+  dd exc_reserved
+  dd exc_reserved
+  dd exc_reserved
+  dd exc_reserved
+  dd exc_reserved
+  dd exc_reserved
+  dd exc_reserved
+  dd exc_reserved
+  dd exc_reserved
+  dd exc_reserved
+  dd exc_reserved
+
+traps_init:
+  push ebp
+  mov ebp, esp
+  mov ecx, 0
+  mov edx, idt
+.loop:
+  mov eax, ecx
+  shl eax, 2
+  add eax, traps
+  mov eax, [eax]
+  mov [edx], ax
+  shr eax, 16
+  mov [edx+6], ax
+  mov word [edx+2], 0x08
+  mov byte [edx+4], 0x00
+  mov byte [edx+5], 0x8E
+  inc ecx
+  cmp ecx, 32
+  je .end
+  add edx, 8
+  jmp .loop
+.end:
+  pop ebp
+  ret
+
+exc_div:
+  SAVE
+  RESTORE
+
+exc_debug:
+  SAVE
+  RESTORE
+
+exc_nmi:
+  SAVE
+  RESTORE
+
+exc_brk:
+  SAVE
+  RESTORE
+
+exc_ovf:
+  SAVE
+  RESTORE
+
+exc_bounds:
+  SAVE
+  RESTORE
+
+exc_invop:
+  SAVE
+  RESTORE
+
+exc_nomath:
+  SAVE
+  RESTORE
+
+exc_dbf:
+  SAVE_ERR
+  RESTORE_ERR
+
+exc_coseg:
+  SAVE
+  RESTORE
+
+exc_invtss:
+  SAVE_ERR
+  RESTORE_ERR
+
+exc_segnotpres:
+  SAVE_ERR
+  RESTORE_ERR
+
+exc_stackf:
+  SAVE_ERR
+  RESTORE_ERR
+
+exc_gprot:
+  SAVE_ERR
+  RESTORE_ERR
+
+exc_pagef:
+  SAVE_ERR
+  RESTORE_ERR
+
+exc_mathf:
+  SAVE
+  RESTORE
+
+exc_alignchk:
+  SAVE
+  RESTORE
+
+exc_machinechk:
+  SAVE
+  RESTORE
+
+exc_simdfp:
+  SAVE
+  RESTORE
+
+exc_virt:
+  SAVE
+  RESTORE
+
+exc_reserved:
+  SAVE
+  RESTORE
+
+syscall_handler:
+  SAVE
+
+  ; pass the saved ESP value to the REAL handler
+  mov eax, [esp+46]
+  push eax
+  call syscall
+  add esp, 4
+
+  RESTORE
+
+
+syscall_init:
+  push ebp
+  mov ebp, esp
+  push syscall_handler
+  push dword 3
+  push dword 0x80
+  call register_isr
+  add esp, 12
+  pop ebp
+  ret
diff --git a/kernel/usrbin/usrbin.bin b/kernel/usrbin/usrbin.bin
index a7d7730..bdf8c29 100644
--- a/kernel/usrbin/usrbin.bin
+++ b/kernel/usrbin/usrbin.bin
diff --git a/kernel/usrbin/usrbin.s b/kernel/usrbin/usrbin.s
index 91dd94a..248d823 100644
--- a/kernel/usrbin/usrbin.s
+++ b/kernel/usrbin/usrbin.s
@@ -3,30 +3,10 @@ org 0x100000
 
 main:
   push .msg
-  call puts
+  push byte 0
+  int 0x80
   add esp, 4
 .loop:
   ; loop forever
   jmp .loop
-.msg: db "Hello World from Userspace!", 0
-
-puts:
-  push ebp
-  mov ebp, esp
-  push esi
-  push edi
-  mov esi, [ebp+8]
-  mov edi, 0xB8000
-.loop:
-  lodsb
-  cmp al, 0
-  je .end
-  mov [edi], al
-  mov byte [edi+1], 0x07
-  add edi, 2
-  jmp .loop
-.end:
-  pop edi
-  pop esi
-  pop ebp
-  ret
+.msg: db "Hello World from Userspace, using syscalls!", 10, 0
diff --git a/kernel/usrspace.s b/kernel/usrspace.s
index e52f2e0..1d20f71 100644
--- a/kernel/usrspace.s
+++ b/kernel/usrspace.s
@@ -1,6 +1,7 @@
 global userspace_init
-extern _binary_usrbin_bin_start
 extern _binary_usrbin_bin_size
+extern _binary_usrbin_bin_start
+extern tss
 
 userspace_init:
   push ebp
@@ -32,8 +33,10 @@ usrcall:
   mov fs, ax
   mov gs, ax
   mov eax, esp
+  ; save ESP in the TSS
+  mov [tss+4], eax
   push dword 0x23
-  push eax
+  push dword 0x00180000
   pushf
   push dword 0x1B
   push dword 0x00100000