BL: Query and store e820 memory map

- bl stage 1: move gdt+pm setup to stage 2 (avoids mode switching)
- bl stage 2: at entry query e820 table from bios, then setup gdt+pm
- kernel/memmap: pad map entry to 24 bytes, as exported by bl
- kernel/memmap: map copy (gbowne1)
- Makefile: facilitate placing the memory map at a known location
- Update bl docs

bootloader/stage2.asm

@@ -1,10 +1,80 @@
-[BITS 32]
 global _start
-global ata_lba_read
-
 extern load_kernel
 
+%define e820_magic 0x534d4150 ; "SMAP"
+%define e820_entry_size 24
+%define e820_max_entries 128
+
+; ----------------------------------------------------------------
+; Real mode
+; ----------------------------------------------------------------
+[BITS 16]
 _start:
+    call read_e820
+    call setup_gdt
+    call switch_to_pm
+
+read_e820:
+    xor ebx, ebx
+    mov es, bx
+    mov di, MEMMAP_BASE+4       ; ES=0 DI=MEMMAP_BASE+4
+    xor bp, bp                  ; Keeping count in bp
+
+.e820_loop:
+    mov eax, 0xe820
+    mov ecx, e820_entry_size
+    mov edx, e820_magic
+    int 0x15
+    jc .done                    ; Error?
+
+    cmp eax, e820_magic         ; Verify "SMAP"
+    jne .done
+
+    test ecx, ecx               ; Skip 0-sized entries
+    jz .skip
+
+    add di, e820_entry_size     ; Advance write addr
+    inc bp                      ; Increment count
+
+    cmp bp, e820_max_entries    ; Stop if we're at capacity
+    jae .done
+.skip:
+    test ebx, ebx
+    jne .e820_loop
+.done:
+    mov [MEMMAP_BASE], bp       ; Store count
+    ret
+
+setup_gdt:
+    lgdt [gdt_descriptor]
+    ret
+
+switch_to_pm:
+    cli
+    mov eax, cr0
+    or eax, 1
+    mov cr0, eax
+    jmp 0x08:pm_entry
+
+e820_count:
+    dw 0
+
+gdt_start:
+    dq 0x0000000000000000         ; Null descriptor
+    dq 0x00CF9A000000FFFF         ; 32-bit code segment (selector 0x08)
+    dq 0x00CF92000000FFFF         ; 32-bit data segment (selector 0x10)
+    dq 0x00009A000000FFFF         ; 16-bit code segment for real mode (selector 0x18)
+gdt_descriptor:
+    dw gdt_end - gdt_start - 1
+    dd gdt_start
+gdt_end:
+
+; ----------------------------------------------------------------
+; Protected mode
+; ----------------------------------------------------------------
+[BITS 32]
+
+pm_entry:
     ; Set up segments
     ; Data segments
     mov ax, 0x10
@@ -18,9 +88,8 @@ _start:
     mov ax, 0x08
     mov cs, ax
 
-    ; Stack (must be identity-mapped)
+    ; Stack
     mov esp, 0x90000
 
     call load_kernel
-
     jmp eax