From fdf6d81b2e5fe2170b445606433a0580388022f7 Mon Sep 17 00:00:00 2001
From: pantonshire <tom@tomandtally.co.uk>
Date: Tue, 23 Jul 2024 07:40:29 +0100
Subject: [PATCH] use e820 to get available memory

---
 boot0.s      |  7 +++--
 boot1.s      | 87 +++++++++++++++++++++++++++++++++++++++++++++++++---
 defines.s    | 14 +++++++--
 resources.md |  1 +
 4 files changed, 99 insertions(+), 10 deletions(-)

diff --git a/boot0.s b/boot0.s
index 0f16081..cce1163 100644
--- a/boot0.s
+++ b/boot0.s
@@ -3,8 +3,9 @@
 ; --------------------------------------------------------------------
 ; R | 0x000000 - 0x000400: real-mode interrupt vector table
 ; R | 0x000400 - 0x000500: bios data area
-; U | 0x000500 - 0x007000: stack
-; U | 0x007000 - 0x007c00: globals
+; U | 0x000500 - 0x004000: stack
+; U | 0x004000 - 0x006a00: globals
+; U | 0x006a00 - 0x007c00: memory map
 ; U | 0x007c00 - 0x007e00: boot sector
 ; U | 0x007e00 - 0x080000: conventional usable memory
 ; R | 0x080000 - 0x0a0000: extended bios data area (maximum possible size)
@@ -29,7 +30,7 @@ main:
   mov ds, ax
   mov es, ax
 
-  ; Put the stack base at 0x7000.
+  ; Put the stack base at 0x4000.
   ; Stack grows high->low, so we'll grow away from our globals and program text.
   mov ss, ax
   mov bp, STACK_BASE
diff --git a/boot1.s b/boot1.s
index 5b61e6e..a3f96f0 100644
--- a/boot1.s
+++ b/boot1.s
@@ -247,8 +247,14 @@ main:
   mov ax, msg_a20_enabled
   call vga_println
   
+  call mem_detect
+  jc .mem_detect_fail
+
   hlt
 
+.mem_detect_fail:
+  panic PANIC_TYPE_MEM_DETECT
+
 
 ; Print a panic message then terminate.
 ; Arguments:
@@ -704,6 +710,74 @@ enable_a20_intel_8042:
   ret
 
 
+mem_detect:
+  %if MEMMAP_CAP < 1
+  %error "memmap buffer too small"
+  %endif
+
+  fnstart
+  push es
+
+  mov word [GLOBALS + MEMMAP_ENTRIES], 0
+
+  ; Memset the memmap buffer to 0.
+  xor ax, ax
+  mov es, ax
+  mov di, MEMMAP
+  mov cx, (MEMMAP_END - MEMMAP)
+  rep stosb
+  
+  ; Entry number (will be incremented for us)
+  xor ebx, ebx
+  ; Buffer (es:di)
+  mov di, MEMMAP
+  mov dword [di + 0x20], 0x1
+  ; Entry size
+  mov ecx, MEMMAP_ENT_SIZE
+  ; Magic number
+  mov edx, E820_MAGIC
+  ; Detect memory
+  mov eax, 0xe820
+  clc
+  int 0x15
+  jc .fail
+  cmp eax, E820_MAGIC
+  jne .fail
+
+  inc word [GLOBALS + MEMMAP_ENTRIES]
+
+.loop:
+  cmp word [GLOBALS + MEMMAP_ENTRIES], MEMMAP_CAP
+  jae .done
+
+  add di, MEMMAP_ENT_SIZE
+  mov dword [di + 0x20], 0x1
+  mov ecx, MEMMAP_ENT_SIZE
+  mov eax, 0xe820
+  clc
+  int 0x15
+  ; Carry flag will be set if we'd already reached the end of the entries.
+  jc .done
+
+  inc word [GLOBALS + MEMMAP_ENTRIES]
+
+  test ebx, ebx
+  jz .done
+
+  jmp .loop
+
+.fail:
+  stc
+  jmp .return
+
+.done:
+  clc
+
+.return:
+  pop es
+  fnret
+
+
 msg_boot1_loaded db "boot1 loaded. hello!", 0
 msg_a20_enabled  db "a20 enabled", 0
 msg_a20_disabled db "a20 not enabled", 0
@@ -746,15 +820,18 @@ table_reg_msgs:
   dw msg_reg_flags
   dw 0
 
-msg_panic_generic db "generic panic", 0
-msg_panic_a20     db "failed to enable a20 line", 0
+msg_panic_generic    db "generic panic", 0
+msg_panic_a20        db "failed to enable a20 line", 0
+msg_panic_mem_detect db "failed to detect available memory", 0
 
 panic_type_msgs:
-  PANIC_TYPE_GENERIC equ ($ - panic_type_msgs) / 2
+  PANIC_TYPE_GENERIC    equ ($ - panic_type_msgs) / 2
   dw msg_panic_generic
-  PANIC_TYPE_A20     equ ($ - panic_type_msgs) / 2
+  PANIC_TYPE_A20        equ ($ - panic_type_msgs) / 2
   dw msg_panic_a20
-  PANIC_TYPE_MAX     equ ($ - panic_type_msgs) / 2
+  PANIC_TYPE_MEM_DETECT equ ($ - panic_type_msgs) / 2
+  dw msg_panic_mem_detect
+  PANIC_TYPE_MAX        equ ($ - panic_type_msgs) / 2
   dw 0
 
 boot1_magic dd BOOT1_MAGIC
diff --git a/defines.s b/defines.s
index 747922e..40d3897 100644
--- a/defines.s
+++ b/defines.s
@@ -1,8 +1,14 @@
 %define BOOT0_LOADPOINT 0x7c00
 %define BOOT1_LOADPOINT 0x8200
 
-%define GLOBALS    0x7000
-%define STACK_BASE GLOBALS
+%define MEMMAP          0x6a00
+%define MEMMAP_END      BOOT0_LOADPOINT
+%define MEMMAP_ENT_SIZE 24
+%define MEMMAP_CAP      ((MEMMAP_END - MEMMAP) / MEMMAP_ENT_SIZE)
+
+%define GLOBALS     0x4000
+%define GLOBALS_END MEMMAP
+%define STACK_BASE  GLOBALS
 
 %define EBDA_START 0x080000
 
@@ -25,6 +31,9 @@
 %define INTEL_8042_CMD_CONTROLLER_OUT_PORT_READ  0xd0
 %define INTEL_8042_CMD_CONTROLLER_OUT_PORT_WRITE 0xd1
 
+; ASCII string "SMAP", which is used by e820 as a magic number argument and return value.
+%define E820_MAGIC 0x534d4150
+
 ; boot0 base stack frame variable offsets / globals
 ; (we use the same offsets once we copy the variables to the globals section)
 ; -------------------------------------------------------------------------------------------------
@@ -53,6 +62,7 @@
 ; ------------------------------------------------------------------------------------------------
 %define VGA_COL                0x1c
 %define TEXTBUF_LINE           0x1e
+%define MEMMAP_ENTRIES         0x20
 
 %macro fnstart 0
   push bp
diff --git a/resources.md b/resources.md
index b1c31a2..32de0dd 100644
--- a/resources.md
+++ b/resources.md
@@ -15,6 +15,7 @@
 ## Memory
 - <https://wiki.osdev.org/Memory_Map_(x86)>
 - <https://wiki.osdev.org/Detecting_Memory_(x86)>
+- <http://www.uruk.org/orig-grub/mem64mb.html>
 
 ## BIOS interrupts
 - <https://en.wikipedia.org/wiki/BIOS_interrupt_call>