idt

README.md

@@ -36,7 +36,6 @@ Minimal operating systems to learn low level programming.
             1.  [int $1](interrupt1.S)
             1.  [Interrupt zero divide](interrupt_zero_divide.S)
             1.  [Interrupt loop](interrupt_loop.S)
-            1.  [lidt (TODO)](lidt.S)
         1.  in
             1.  [in keyboard](in_keyboard.S)
             1.  [in RTC](in_rtc.S)
@@ -45,7 +44,9 @@ Minimal operating systems to learn low level programming.
             1.  [in beep_illinois](in_beep_illinois.S)
             1.  [in mouse (TODO)](in_mouse.S)
         1.  [Protected mode](protected_mode.S)
-            1. [Segment base](segment_base.S)
+            1. [Segment base (TODO)](segment_base.S)
+            1. [IDT TODO](idt.S)
+                1. [IDT zero divide](idt_zero_divide.S)
             1. Segmentation fault handler: memory bound, ring, RWX violations
     1.  APM
         1.  [APM shutdown](apm_shutdown.S)

TODO.md

@@ -114,4 +114,13 @@
 
 -   mouse
 
-    - DOS question: http://stackoverflow.com/questions/23043732/accessing-the-mouse-via-assembly-x86
+    Does not seem to be an easy BIOS way:
+
+    -   DOS question: http://stackoverflow.com/questions/23043732/accessing-the-mouse-via-assembly-x86
+
+    -   http://stackoverflow.com/questions/5754233/int-33h-doesnt-work
+
+    -   Linux questions:
+
+        - http://stackoverflow.com/questions/15322892/linux-usb-mouse-drivers
+        - http://stackoverflow.com/questions/25175960/which-drivers-are-used-by-usb-mouse-in-linux-kernel

common.h

@@ -53,6 +53,20 @@ so this cancels that one.
     pop %ax
 .endm
 
+.macro PUSH_EADX
+    push %eax
+    push %ebx
+    push %ecx
+    push %edx
+.endm
+
+.macro POP_EDAX
+    pop %edx
+    pop %ecx
+    pop %ebx
+    pop %eax
+.endm
+
 /*
 Convert the low nibble of a r8 reg to ASCII of 8-bit in-place.
 reg: r8 to be converted
@@ -155,7 +169,6 @@ Use the simplest GDT possible.
     .equ CODE_SEG, 8
     .equ DATA_SEG, gdt_data - gdt_start
 
-    cli
     /* Tell the processor where our Global Descriptor Table is in memory. */
     lgdt gdt_descriptor
 
@@ -177,6 +190,9 @@ Our GDT contains:
   - it is impossible execute the data segment
   Both start at 0 and span the entire memory,
   allowing us to access anything without problems.
+A real OS might have 2 extra segments: user data and code.
+This is better than modifying the privilege bit of the GDT
+as we'd have to reload it several times, losing cache.
 */
 gdt_start:
 gdt_null:
@@ -372,10 +388,7 @@ Loops around the to the top.
 */
 .macro VGA_PRINT_STRING s
     LOCAL loop, end
-    push %eax
+    PUSH_EADX
-    push %ebx
-    push %ecx
-    push %edx
     mov \s, %ecx
     mov vga_current_line, %eax
     mov $0, %edx
@@ -400,10 +413,7 @@ loop:
     jmp loop
 end:
     incl vga_current_line
-    pop %edx
+    POP_EDAX
-    pop %ecx
-    pop %ebx
-    pop %eax
 .endm
 
 /*
@@ -420,10 +430,7 @@ Expected output on screen:
 */
 .macro VGA_PRINT_REG reg=<%eax>
     LOCAL loop
-    push %eax
+    PUSH_EADX
-    push %ebx
-    push %ecx
-    push %edx
     /* Null terminator. */
     mov \reg, %ecx
 
@@ -448,8 +455,5 @@ loop:
 
     /* Restore the stack. We have pushed 3 * 4 bytes. */
     add $12, %esp
-    pop %edx
+    POP_EDAX
-    pop %ecx
-    pop %ebx
-    pop %eax
 .endm

debug.md

@@ -6,7 +6,7 @@ This will only cover specifics, you have to know GDB debugging already.
 
 First read the `make debug` target to get started. Those points will not be repeated here.
 
-How to have debug symbols: <http://stackoverflow.com/a/32960272/895245> TODO implement here.
+How to have debug symbols: <http://stackoverflow.com/a/32960272/895245> TODO implement here. Needs to point GDB to an ELF file in addition to the remote listen.
 
 How to step over `int` calls: <http://stackoverflow.com/questions/24491516/how-to-step-over-interrupt-calls-when-debugging-a-bootloader-bios-with-gdb-and-q>
 

idt.S

@@ -0,0 +1,63 @@
+/*
+# IDT
+
+# Interrupt Descriptor Table
+
+Expected output: "a"
+
+The first 32 handlers are reserved by the processor and have predefined meanings.
+
+0 is division by zero.
+
+TODO on Linux kernel.
+
+## lidt
+
+Analogous to lgdt but for the IDT.
+
+## Bibliography
+
+- http://www.jamesmolloy.co.uk/tutorial_html/4.-The%20GDT%20and%20IDT.html
+*/
+
+#include "common.h"
+
+BEGIN
+    STAGE2
+    CLEAR
+    PROTECTED_MODE
+    lidt idt_descriptor
+    int $0
+    jmp .
+idt_start:
+    /* Low handler address. */
+    /* TODO get this proper version working. */
+    /*.word (handler & 0x0000FFFF)*/
+    .word handler
+    /* Segment selector: byte offset into the GDT. */
+    .word CODE_SEG
+    /* Reserved 0. */
+    .byte 0
+    /*
+    Flags. Format:
+    - 1 bit: present. If 0 and this happens, triple fault.
+    - 2 bits: ring level we will be called from.
+    - 5 bits: fixed to 0xE.
+    */
+    .byte 0x8E
+    /* High word of base. */
+    /* TODO get this proper version working. */
+    /*.word (handler >> 16) & 0xFFFF*/
+    .word 0
+idt_end:
+/* Exact same structure as gdt_descriptor. */
+idt_descriptor:
+    .word idt_end - idt_start
+    .long idt_start
+handler:
+    /* TODO needed? Molloy uses it. */
+    /*cli*/
+    VGA_PRINT_STRING $message
+    iret
+message:
+    .asciz "a"

idt_zero_divide.S

@@ -0,0 +1,34 @@
+/*
+# IDT Zero Divide
+
+TODO why does it enter an infinite loop? How to prevent it?
+
+Division by zero causes a Divide Error which Intel notes as `#DE`.
+*/
+
+#include "common.h"
+
+BEGIN
+    STAGE2
+    CLEAR
+    PROTECTED_MODE
+    lidt idt_descriptor
+    mov $0, %eax
+    div %eax
+    jmp .
+/* TODO DRY up with idt.S */
+idt_start:
+    .word handler
+    .word CODE_SEG
+    .byte 0
+    .byte 0x8E
+    .word 0
+idt_end:
+idt_descriptor:
+    .word idt_end - idt_start
+    .long idt_start
+handler:
+    VGA_PRINT_STRING $message
+    iret
+message:
+    .asciz "a"

interrupt.S

@@ -24,7 +24,7 @@ before the interrupt came in.
 I think this is mandatory, e.g. a `jmp` wouldn't be enough because:
 
 - we may have far jumped
-- iret also pops EFLAGS restoring
+- iret also pops EFLAGS restoring. TODO more things also seem restored: CS, EIP, EFLAGS, SS, and ESP
 
 ## ISR
 

interrupt_zero_divide.S

@@ -9,8 +9,7 @@ BEGIN
     movw %cs, 0x02
     mov $0, %ax
     div %ax
-    jmp fail
+    hlt
 handler:
     PUTC $'a
-fail:
+    iret
-    hlt