From 51b654d5e354f4784925e232f88e02b4dc0168c9 Mon Sep 17 00:00:00 2001
From: "Pierre-Emmanuel Gob." <pierre.emmanuel.gobillard@gmail.com>
Date: Mon, 5 Oct 2026 18:11:57 +0200
Subject: [PATCH] entry: copy staged E820 map into .bss memmap_*

---
 include/e820.h               | 34 ++++++++++++++++++++++++++++++++++
 kernel/arch/x86_64/entry.asm | 33 ++++++++++++++++++++++++++++++++-
 2 files changed, 66 insertions(+), 1 deletion(-)
 create mode 100644 include/e820.h

diff --git a/include/e820.h b/include/e820.h
new file mode 100644
index 0000000..9f87c08
--- /dev/null
+++ b/include/e820.h
@@ -0,0 +1,34 @@
+/* Nadir BIOS E820 memory-map entry (int 0x15, eax=0xE820).
+ *
+ * Staged by the stage-1 boot sector in 16-bit real mode at Ring 0 (CPL 0)
+ * into low memory (0x5000 count, 0x5020 entries) while BIOS calls still
+ * work, then copied by the stage-2 entry into the .bss globals
+ * memmap_count / memmap_entries owned by the PMM. Runs under the
+ * identity-mapped first 1GB, so physical == virtual addresses here.
+ * Fault behavior: no IDT exists during staging/copy, so a bad pointer
+ * triple-faults; the entry count is clamped to E820_MAX on both ends.
+ */
+
+#ifndef NADIR_E820_H
+#define NADIR_E820_H
+
+#include <stdint.h>
+
+/* Max entries the boot sector stages and the kernel keeps. */
+#define E820_MAX 64
+
+/* Usable-RAM type in the E820 type field. */
+#define E820_USABLE 1
+
+/* One 24-byte BIOS E820 entry: byte-based [base, base+length) range.
+ * Preconditions: filled by BIOS int 0x15 or by the boot-sector fallback
+ * (single type-1 region at 1MB); length may be 0 (ignored by the PMM).
+ * Failure modes: none in C — a corrupt table only mis-marks frames. */
+struct e820_entry {
+    uint64_t base;
+    uint64_t length;
+    uint32_t type;
+    uint32_t acpi_attr;
+} __attribute__((packed));
+
+#endif /* NADIR_E820_H */
diff --git a/kernel/arch/x86_64/entry.asm b/kernel/arch/x86_64/entry.asm
index 34e73db..47a87b0 100644
--- a/kernel/arch/x86_64/entry.asm
+++ b/kernel/arch/x86_64/entry.asm
@@ -9,8 +9,14 @@
 ;
 ; Steps: verify long mode exists, identity-map the first 1GB with 2MB
 ; pages, enable PAE + LME + paging, reload a GDT with a 64-bit code
-; segment, zero BSS, then call the C entry point kmain (System V ABI).
+; segment, zero BSS, copy the BIOS memory map staged by stage 1 into
+; kernel .bss, then call the C entry point kmain (System V ABI).
 ; Interrupts stay disabled for the whole sequence.
+;
+; WHY copy after zeroing: the map globals live in .bss, so zeroing first
+; then copying keeps the staged entries. Zeroing after would wipe them;
+; copying before is impossible (BSS addresses are only valid once paging
+; and the zeroed state the C code expects are both in place).
 
 bits 32
 section .boot32
@@ -18,11 +24,17 @@ global boot32_entry
 extern kmain
 extern __bss_start
 extern __bss_end
+extern memmap_count
+extern memmap_entries
 
 PML4        equ 0x70000
 PDPT        equ 0x71000
 PD          equ 0x72000
 STACK64_TOP equ 0x90000
+MEMMAP_COUNT_PHYS equ 0x5000        ; dword staged by stage 1
+MEMMAP_BUF_PHYS   equ 0x5020        ; 24-byte E820 entries staged by stage 1
+E820_MAX          equ 64
+E820_ENTRY_SIZE   equ 24
 
 boot32_entry:
     ; --- CPUID: does this CPU do 64-bit long mode? ---
@@ -121,6 +133,25 @@ long_entry:
     xor eax, eax
     rep stosb
 
+    ; --- copy staged E820 map into .bss globals (identity map covers
+    ; both 0x5000 and .bss, and paging is already on) ---
+    ; WHY byte + clamp: stage 1 stages a single count byte (64 max fits);
+    ; the clamp is belt-and-braces against a corrupt staging byte.
+    movzx eax, byte [abs MEMMAP_COUNT_PHYS]
+    cmp eax, E820_MAX
+    jbe .map_count_ok
+    mov eax, E820_MAX
+.map_count_ok:
+    mov [rel memmap_count], eax
+    test eax, eax
+    jz .map_done
+    mov rsi, MEMMAP_BUF_PHYS
+    lea rdi, [rel memmap_entries]
+    mov ecx, eax
+    imul ecx, ecx, E820_ENTRY_SIZE  ; bytes = count * 24
+    rep movsb
+.map_done:
+
     call kmain                      ; first C code — does not return
     cli
 .hang: