DRY up IDT

about.md

@@ -12,6 +12,8 @@ Minimal examples are useful because it is easier to observe the requirements for
 
 Another advantage is that it is easier to DRY up minimal examples (here done simply through `#include` and macros), which is much harder on progressive OS template tutorials, which tend to repeat big chunks of code between the examples.
 
+Using C or not is a hard choice. It does make it much easier to express higher level ideas, and gives portability. But in the end, it increases the complexity that one has to understand, so we've stayed away from it.
+
 ## Pre-requisites
 
 OS dev is one of the most insanely hard programming tasks a person can undertake, and will push your knowledge of several domains to the limit.

common.h

@@ -457,3 +457,52 @@ loop:
     add $12, %esp
     POP_EDAX
 .endm
+
+.macro IDT_START
+    idt_start:
+.endm
+
+.macro IDT_END
+idt_end:
+/* Exact same structure as gdt_descriptor. */
+idt_descriptor:
+    .word idt_end - idt_start
+    .long idt_start
+.endm
+
+.macro IDT_ENTRY
+    /* Low handler address.
+    It is impossible to write:
+    .word (handler & 0x0000FFFF)
+    as we would like:
+    http://stackoverflow.com/questions/18495765/invalid-operands-for-binary-and
+    So this must be done at runtime.
+    */
+    .word 0
+    /* 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. */
+    .word 0
+.endm
+
+/*
+- index: r/m/imm32 Index of the entry to setup.
+- handler: r/m/imm32 Address of the handler function.
+*/
+.macro IDT_SETUP_ENTRY index, handler
+    mov \index, %eax
+    mov \handler, %ebx
+    mov %bx, idt_start(%eax, 8)
+    shr $16, %ebx
+    mov $6, %ecx
+    mov %bx, idt_start(%ecx, %eax, 8)
+.endm

idt.S

@@ -7,8 +7,6 @@ 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
@@ -26,34 +24,13 @@ BEGIN
     STAGE2
     CLEAR
     PROTECTED_MODE
+    IDT_SETUP_ENTRY $0, $handler
     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
+IDT_START
+IDT_ENTRY
+IDT_END
 handler:
     /* TODO needed? Molloy uses it. */
     /*cli*/

idt_zero_divide.S

@@ -3,6 +3,10 @@
 
 Division by zero causes a Divide Error which Intel notes as `#DE`.
 
+Expected output: "a"
+
+It is then handled by IDT 0.
+
 Remember that DE is not *only* for division by zero: it also happens on overflow!
 Thus the name: Division Error, and not Division by zero.
 
@@ -15,6 +19,7 @@ BEGIN
     STAGE2
     CLEAR
     PROTECTED_MODE
+    IDT_SETUP_ENTRY $0, $handler
     lidt idt_descriptor
     mov $0, %edx
     mov $1, %eax
@@ -22,17 +27,9 @@ BEGIN
     /* The iret jumps back here! */
     div %ecx
     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
+IDT_START
+IDT_ENTRY
+IDT_END
 handler:
     VGA_PRINT_STRING $message
     /* If we don't do this, we get an infinite loop! */