From d15609e5c008dcd76f6b02ecd8a3ba9f7566083a Mon Sep 17 00:00:00 2001
From: nub31 <oliverlhs@outlook.com>
Date: Tue, 30 Dec 2025 22:25:51 +0100
Subject: [PATCH] ...

---
 src/boot/main.c    | 20 +++++++++++-------
 src/boot/start.asm | 51 ++++++++++++++++++++++++++++++++--------------
 src/boot/util.h    |  8 ++++++--
 3 files changed, 55 insertions(+), 24 deletions(-)

diff --git a/src/boot/main.c b/src/boot/main.c
index 8949380..0cccffc 100644
--- a/src/boot/main.c
+++ b/src/boot/main.c
@@ -21,7 +21,7 @@ static void *multiboot_find_tag(uptr multiboot_info, u32 type) {
             return tag;
         }
 
-        next = align(next + tag->size, MULTIBOOT_TAG_ALIGN);
+        next = align_up(next + tag->size, MULTIBOOT_TAG_ALIGN);
     }
 }
 
@@ -30,13 +30,13 @@ static void *multiboot_find_tag(uptr multiboot_info, u32 type) {
 typedef struct {
     u64 base_address;
     u64 length;
-} region;
+} memory_region;
 
 extern uptr kernel_start;
 extern uptr kernel_end;
 
-static region regions[MAX_REGIONS] = {0};
-static size_t region_count = 0;
+static memory_region memory_regions[MAX_REGIONS] = {0};
+static size_t memory_region_count = 0;
 
 static void find_memory_regions(uptr multiboot_info) {
     multiboot_tag_mmap *tag = multiboot_find_tag(multiboot_info, MULTIBOOT_TAG_TYPE_MMAP);
@@ -51,17 +51,23 @@ static void find_memory_regions(uptr multiboot_info) {
         multiboot_mmap_entry *entry = (multiboot_mmap_entry*)entry_ptr;
 
         if (entry->type == MULTIBOOT_MEMORY_AVAILABLE) {
-            if (region_count >= MAX_REGIONS) {
+            if (memory_region_count >= MAX_REGIONS) {
                 boot_panic("Too many memory regions");
             }
 
-            regions[region_count++] = (region){ entry->base_address, entry->length };
+            memory_regions[memory_region_count++] = (memory_region){ entry->base_address, entry->length };
         }
 
         entry_ptr += tag->entry_size;
     }
 }
 
-void kmain(uptr multiboot_info) {
+// We are now in long mode with kernel pages and vga buffer identity mapped
+void x86_64_main(uptr multiboot_info) {
+    console_clear();
     find_memory_regions(multiboot_info);
+
+    for (u32 i = 0; i < memory_region_count; ++i) {
+        kprintf("region: base_address=0x%X, length=0x%X\n", memory_regions[i].base_address, memory_regions[i].length);
+    }
 }
\ No newline at end of file
diff --git a/src/boot/start.asm b/src/boot/start.asm
index fcddbf3..8a4d6a7 100644
--- a/src/boot/start.asm
+++ b/src/boot/start.asm
@@ -1,7 +1,7 @@
 global _start
 extern kernel_start
 extern kernel_end
-extern kmain
+extern x86_64_main
 
 %define MAGIC   0xe85250d6
 %define ARCH    0x0
@@ -21,10 +21,6 @@ header:
         dd 8
     .end:
 
-section .data
-align 8
-multiboot_info: dq 0
-
 %define PRESENT     1 << 7
 %define NOT_SYS     1 << 4
 %define EXEC        1 << 3
@@ -58,6 +54,11 @@ gdt:
         dq gdt
 
 section .bss
+%define MULTIBOOT_INFO_MAX_SIZE 1024 * 64
+
+align 8
+multiboot_info: resb MULTIBOOT_INFO_MAX_SIZE
+
 align 16
 stack:
     .bottom:
@@ -79,29 +80,38 @@ _start:
     cmp eax, BOOTLOADER_MAGIC
     jne .halt
 
-    ; Save pointer to info gathered by multiboot
-    mov [multiboot_info], ebx
+    ; Copy multiboot structure to kernel owned memory
+    mov esi, ebx
+    mov eax, [esi]
+
+    cmp eax, MULTIBOOT_INFO_MAX_SIZE
+    ja .halt
+
+    mov edi, multiboot_info
+    mov ecx, eax
+    cld
+    rep movsb
 
     ; Tell the cpu where the pages area
     mov edi, pml4t
     mov cr3, edi
 
-    %define PT_PRESENT      1
-    %define PT_READABLE     2
+    %define PT_PRESENT  1
+    %define PT_RW       2
 
     ; pml4t[0] -> pdpt
     mov edi, pdpt
-    or edi, PT_PRESENT | PT_READABLE
+    or edi, PT_PRESENT | PT_RW
     mov [pml4t], edi
 
     ; pdpt[0] -> pdt
     mov edi, pdt
-    or edi, PT_PRESENT | PT_READABLE
+    or edi, PT_PRESENT | PT_RW
     mov [pdpt], edi
 
     ; pdt[0] -> pt
     mov edi, pt
-    or edi, PT_PRESENT | PT_READABLE
+    or edi, PT_PRESENT | PT_RW
     mov [pdt], edi
 
     %define ENTRIES_PER_PT  512
@@ -113,7 +123,7 @@ _start:
     mov edi, pt
     mov ebx, kernel_start
     and ebx, 0xfffff000
-    or ebx, PT_PRESENT | PT_READABLE
+    or ebx, PT_PRESENT | PT_RW
 
     mov eax, kernel_end
     sub eax, kernel_start
@@ -127,6 +137,17 @@ _start:
     add edi, SIZEOF_PT_ENTRY
     loop .set_pt_entry
 
+    ; note(nub31): Vga buffer is already page aligned
+    %define VGA_BUFFER 0xb8000
+
+    ; Identity map vga buffer
+    mov ebx, VGA_BUFFER
+    or ebx, PT_PRESENT | PT_RW
+
+    mov edi, pt
+    add edi, VGA_BUFFER / PAGE_SIZE * SIZEOF_PT_ENTRY
+    mov [edi], ebx
+
     %define CR4_PAE_ENABLE  1 << 5
 
     ; Enable pae
@@ -169,8 +190,8 @@ start64:
     mov ss, ax
 
     mov rsp, stack.top
-    mov rdi, [multiboot_info]
-    call kmain
+    mov rdi, multiboot_info
+    call x86_64_main
 .halt:
     cli
     hlt
diff --git a/src/boot/util.h b/src/boot/util.h
index 361ca39..75d41e4 100644
--- a/src/boot/util.h
+++ b/src/boot/util.h
@@ -1,7 +1,11 @@
 #pragma once
 
-static inline u32 align(u32 num, u32 alignment) {
-    return (num + alignment - 1) & ~(alignment - 1);
+static inline uptr align_up(uptr value, uptr alignment) {
+    return (value + alignment - 1) / alignment * alignment;
+}
+
+static inline uptr align_down(uptr value, uptr alignment) {
+    return value - (value % alignment);
 }
 
 static inline void hlt() {