Implement low-level CPU initialization support for i686 and AMD64 boot sectors

2025-10-17 09:05:24 +02:00
parent ca8a539c0e
commit 764fec4d75
2 changed files with 266 additions and 0 deletions
--- a/boot/bootsect/i686/cpu.S
+++ b/boot/bootsect/i686/cpu.S
@@ -0,0 +1,123 @@
+/**
+* PROJECT:         ExectOS
+* COPYRIGHT:       See COPYING.md in the top level directory
+* FILE:            boot/bootsect/i686/cpu.S
+* DESCRIPTION:     Low-level support for CPU initialization
+* DEVELOPERS:      Aiken Harris <harraiken91@gmail.com>
+*/
+
+
+BuildPageMap:
+    /* Generate page map for first 16MB of memory */
+    pushaw
+    pushw %es
+    cld
+    movw $(0x1000 >> 0x04), %ax
+    movw %ax, %es
+    xorw %di, %di
+    movl $(0x2000 | 0x03), %eax
+    stosl
+    movl $(0x3000 | 0x03), %eax
+    stosl
+    movl $(0x4000 | 0x03), %eax
+    stosl
+    movl $(0x5000 | 0x03), %eax
+    stosl
+    xorl %eax, %eax
+    movw $(1024 - 4), %cx
+    rep stosl
+    movl $0x00000003, %eax
+    movl $4, %edx
+    movw $(0x2000 >> 0x04), %bx
+.BuildPageMapLoop:
+    /* Identity map 1024 pages of 4KB */
+    movw %bx, %es
+    xorw %di, %di
+    pushl %edx
+    movw $1024, %cx
+.FillPageMapTable:
+    /* Fill the page table */
+    movl %eax, %es:(%di)
+    addl $4096, %eax
+    addw $0x04, %di
+    loop .FillPageMapTable
+    popl %edx
+    addw $(0x1000 >> 0x04), %bx
+    decl %edx
+    jnz .BuildPageMapLoop
+    popw %es
+    popaw
+    ret
+
+InitializeCpu:
+    /* Check if CPU supports CPUID */
+    pushal
+    pushfl
+    popl %eax
+    movl %eax, %ebx
+    xorl $0x00200000, %eax
+    pushl %eax
+    popfl
+    pushfl
+    popl %eax
+    cmpl %ebx, %eax
+    je CpuUnsupported
+    popal
+    ret
+
+LoadGdt:
+    /* Load Global Descriptor Table */
+    lgdt .GdtPointer
+    ret
+
+RunStage2:
+    /* Switch to protected mode and pass control to Stage 2 */
+    call BuildPageMap
+    call ParseExecutableHeader
+    pushl %eax
+    cli
+    movl %cr0, %eax
+    orl $0x01, %eax
+    movl %eax, %cr0
+    ljmp $0x08, $.Stage2ProtectedMode
+.code32
+.Stage2ProtectedMode:
+    /* Set segments and stack, then jump to Stage 2 */
+    movw $0x10, %ax
+    movw %ax, %ds
+    movw %ax, %es
+    movw %ax, %ss
+    xorw %ax, %ax
+    movw %ax, %fs
+    movw %ax, %gs
+    popl %eax
+    xorl %ebx, %ebx
+    xorl %ecx, %ecx
+    xorl %edx, %edx
+    xorl %esi, %esi
+    xorl %edi, %edi
+    xorl %ebp, %ebp
+    movl $0x1000, %ebx
+    movl %ebx, %cr3
+    movl %cr0, %ebx
+    orl $0x80000000, %ebx
+    movl %ebx, %cr0
+    jmp *%eax
+
+.code16
+.GdtDescriptor:
+    /* Global Descriptor Table */
+    .quad 0x0000000000000000
+    .quad 0x00CF9A000000FFFF
+    .quad 0x00CF92000000FFFF
+    .quad 0x00009E000000FFFF
+    .quad 0x000092000000FFFF
+
+.GdtPointer:
+    /* Pointer to Global Descriptor Table */
+    .word .GdtPointer - .GdtDescriptor - 1
+    .long .GdtDescriptor
+
+.Stage2FileName:
+    /* Name of Stage 2 executable file */
+    .ascii "BOOTIA32EFI"