Merge pull request #109 from vmttmv/bl-e820

BL: Query and store e820 memory map
2026-02-11 13:25:19 -08:00 · 2026-02-02 07:49:09 -08:00
parent 9be6a4e64b 4fa82854dd
commit 56faa3143d
6 changed files with 98 additions and 53 deletions
--- a/9
+++ b/9
@@ -8,8 +8,13 @@ OBJCOPY = i386-elf-objcopy
 BUILD_DIR = build
 CROSS_DIR = cross
 DISK_IMG = $(BUILD_DIR)/disk.img
+
+STAGE2_ADDR = 0x7e00
 STAGE2_SIZE = 2048

+# Place the memory map (e820) past stage2 bl in memory
+MEMMAP_BASE = $(shell echo $$(($(STAGE2_ADDR) + $(STAGE2_SIZE))))
+
 KERNEL_C_SRC = $(wildcard kernel/*.c)
 KERNEL_ASM_SRC = $(wildcard kernel/*.asm)
 KERNEL_OBJ = $(patsubst kernel/%.c, $(BUILD_DIR)/%.o, $(KERNEL_C_SRC))
@@ -29,7 +34,7 @@ stage1: $(BUILD_DIR)
 # NOTE: Stage2 final size should be checked against `$(STAGE2_SIZE)` by the build system to avoid an overflow.
 # Alternatively, convey the final stage2 size through other means to stage1.
 stage2: $(BUILD_DIR)
-	$(AS) $(ASFLAGS) -o $(BUILD_DIR)/stage2.o bootloader/stage2.asm
+	$(AS) $(ASFLAGS) -DMEMMAP_BASE=$(MEMMAP_BASE) -o $(BUILD_DIR)/stage2.o bootloader/stage2.asm
 	$(CC) -std=c11 -ffreestanding -nostdlib -nostdinc -fno-stack-protector -m32 -Iklibc/include -g -c -o $(BUILD_DIR)/stage2_load.o bootloader/stage2_load.c
 	$(LD) -Tbootloader/stage2.ld -melf_i386 -o $(BUILD_DIR)/$@.elf $(BUILD_DIR)/stage2.o $(BUILD_DIR)/stage2_load.o
 	$(OBJCOPY) -O binary $(BUILD_DIR)/$@.elf $(BUILD_DIR)/$@.bin
@@ -39,7 +44,7 @@ $(BUILD_DIR)/asm_%.o: kernel/%.asm
 	$(AS) $(ASFLAGS) -o $@ $<

 $(BUILD_DIR)/%.o: kernel/%.c
-	$(CC) -std=c11 -ffreestanding -nostdlib -nostdinc -fno-stack-protector -m32 -Iklibc/include -g -c -o $@ $<
+	$(CC) -DMEMMAP_BASE=$(MEMMAP_BASE) -std=c11 -ffreestanding -nostdlib -nostdinc -fno-stack-protector -m32 -Iklibc/include -g -c -o $@ $<

 $(BUILD_DIR)/klibc/%.o: klibc/src/%.c
 	$(CC) -std=c11 -ffreestanding -nostdlib -nostdinc -fno-stack-protector -m32 -Iklibc/include -g -c -o $@ $<
--- a/bootloader/README.md
+++ b/bootloader/README.md
@@ -11,16 +11,16 @@ Bootloader documentation for ClassicOS

 ## Stage 1 (`stage1.asm`)

-Responsible for loading the second stage using BIOS routines, and switching to protected mode.
-
 - Queries CHS parameters from BIOS
 - Loads the second stage bootloader (2048 B) to `0x7c00`
- Sets up a GDT with descriptor entries for code and data both covering the whole 32-bit address space
 - Enables A20
- Set CR0.PE (enable protected mode) and jump to stage 2
+- Jumps to stage2

 ## Stage 2 (`stage2.asm, stage2_load.c`)

+- Read and store E820 memory map from BIOS
+- Sets up a GDT with descriptor entries for code and data both covering the whole 32-bit address space
+- Set CR0.PE (enable protected mode)
 - Set up segment registers
 - Load the kernel ELF header
 - Parse the program headers, and load all `PT_LOAD` segments from disk
--- a/bootloader/stage1.asm
+++ b/bootloader/stage1.asm
@@ -40,11 +40,8 @@ _start:
    call enable_a20
    jc a20_error                ; Jump if A20 enable fails

-    ; Setup Global Descriptor Table
-    call setup_gdt
-
-    ; Switch to protected mode and jump to second stage at 0x08:0x7E00
-    call switch_to_pm
+    ; Jump to s2
+    jmp 0x7e00

 disk_error:
    mov si, disk_error_msg
@@ -241,30 +238,6 @@ check_a20:
    clc                      ; Clear carry flag to indicate success
    ret

-; ----------------------------------------------------------------
-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:
-
-setup_gdt:
-    lgdt [gdt_descriptor]
-    ret
-
-; ----------------------------------------------------------------
-switch_to_pm:
-    cli
-    mov eax, cr0
-    or eax, 1
-    mov cr0, eax
-    jmp 0x08:0x7E00             ; jump to S2
-
 ; ----------------------------------------------------------------
 print_string_16:
 .loop:
--- a/bootloader/stage2.asm
+++ b/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
--- a/kernel/memmap.c
+++ b/kernel/memmap.c
@@ -1,19 +1,16 @@
 #include "memmap.h"

+#define BOOTLOADER_MEMMAP_COUNT_ADDR    MEMMAP_BASE
+#define BOOTLOADER_MEMMAP_ADDR          (MEMMAP_BASE + 4)
+
 uint32_t get_memory_map(memory_map_entry_t *map, uint32_t max_entries) {
+    // Read the number of entries found by the bootloader
+    uint32_t entries_found = *(uint32_t*)BOOTLOADER_MEMMAP_COUNT_ADDR;
+    memory_map_entry_t *bios_data = (memory_map_entry_t*)BOOTLOADER_MEMMAP_ADDR;
+
    uint32_t count = 0;
-
-    if (max_entries >= 1) {
-        map[count].base_addr = 0x00000000;
-        map[count].length = 0x0009FC00;
-        map[count].type = 1;
-        count++;
-    }
-
-    if (max_entries >= 2) {
-        map[count].base_addr = 0x00100000;
-        map[count].length = 0x1FF00000;
-        map[count].type = 1;
+    while (count < entries_found && count < max_entries) {
+        map[count] = bios_data[count];
        count++;
    }

--- a/kernel/memmap.h
+++ b/kernel/memmap.h
@@ -7,6 +7,7 @@ typedef struct {
    uint64_t base_addr;
    uint64_t length;
    uint32_t type;
+    uint32_t ext;
 } __attribute__((packed)) memory_map_entry_t;

 uint32_t get_memory_map(memory_map_entry_t *map, uint32_t max_entries);