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https://github.com/gbowne1/ClassicOS.git
synced 2026-01-15 02:25:19 -08:00
fixed up some bad comments in boot.asm
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@@ -16,10 +16,10 @@ identify_drive:
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cmp cl, 0x80 ; Check for hard disk interrupt vector (example)
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je is_harddrive
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; Handle invalid drive type (error handling)
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...
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; ...
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is_floppy:
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Perform floppy disk access (assuming AH=0x02 for read sectors)
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; Perform floppy disk access (assuming AH=0x02 for read sectors)
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mov ah, 0x02 ; Read sectors
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mov al, 1 ; Number of sectors to read (1)
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@@ -68,8 +68,8 @@ memory_error:
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; ... (error handling or continue with limited memory)
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; Second stage loading (simplified example)
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Here's an improved version of the load_second_stage section with the placeholder jump replaced by actual loading logic:
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Code snippet
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; Here's an improved version of the load_second_stage section with the placeholder jump replaced by actual loading logic:
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; Code snippet
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load_second_stage:
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; Calculate address of second stage bootloader (assuming offset from boot sector)
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@@ -8,89 +8,82 @@ start:
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MOV SP, 0xFFFF ; Stack grows downwards from the top of the segment
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; Copy boot sector data to a safe location
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mCopyBootSector
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call mCopyBootSector
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; Load the kernel
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CALL load_kernel
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switch_to_protected_mode:
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CLI ; Disable interrupts
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lgdt [gdt_descriptor] ; Load the global descriptor table
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MOV EAX, CR0
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OR EAX, 0x1 ; Set the PE (Protection Enable) bit
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MOV CR0, EAX
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; Far jump to flush CPU queue after changing to protected mode
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JMP CODE_SEG:init_pm ; CODE_SEG is the segment selector for code segment in GDT
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init_pm:
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; Update segment registers here
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; Set code segment register (CS) to point to code segment descriptor (selector 1)
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mov ax, 0x0001
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mov ds, ax ; Set data segment register (DS) to point to data segment descriptor (selector 2)
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mov es, ax ; Set other segment registers (ES, SS, etc.) as needed
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RET
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switch_to_protected_mode:
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CLI ; Disable interrupts
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lgdt [gdt_descriptor] ; Load the global descriptor table
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MOV EAX, CR0
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OR EAX, 0x1 ; Set the PE (Protection Enable) bit
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MOV CR0, EAX
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; Far jump to flush CPU queue after changing to protected mode
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JMP CODE_SEG:init_pm ; CODE_SEG is the segment selector for code segment in GDT
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init_pm:
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; Update segment registers here
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; Set code segment register (CS) to point to code segment descriptor (selector 1)
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mov ax, 0x0001
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mov ds, ax ; Set data segment register (DS) to point to data segment descriptor (selector 2)
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mov es, ax ; Set other segment registers (ES, SS, etc.) as needed
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RET
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enable_a20:
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cli ; Disable interrupts to prevent interference
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call a20wait ; Wait for the keyboard controller to be ready
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mov al, 0xAD ; Command to disable keyboard
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out 0x64, al ; Send command to keyboard controller command port
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call a20wait ; Wait for the keyboard controller to be ready
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mov al, 0xD0 ; Command to read output port
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out 0x64, al ; Send command to keyboard controller command port
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call a20wait ; Wait for the keyboard controller to be ready
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in al, 0x60 ; Read current state of output port
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or al, 0x02 ; Set A20 bit
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out 0x64, al ; Send command to keyboard controller command port
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call a20wait ; Wait for the keyboard controller to be ready
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mov al, 0xD1 ; Command to write output port
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out 0x64, al ; Send command to keyboard controller command port
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call a20wait ; Wait for the keyboard controller to be ready
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mov al, 0xAE ; Command to re-enable keyboard
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out 0x64, al ; Send command to keyboard controller command port
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sti ; Re-enable interrupts
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ret
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enable_a20:
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; Enable A20 gate
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cli ; Disable interrupts to prevent interference
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call a20wait ; Wait for the keyboard controller to be ready
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mov al, 0xAD ; Command to disable keyboard
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out 0x64, al ; Send command to keyboard controller command port
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call a20wait ; Wait for the keyboard controller to be ready
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mov al, 0xD0 ; Command to read output port
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out 0x64, al ; Send command to keyboard controller command port
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call a20wait ; Wait for the keyboard controller to be ready
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in al, 0x60 ; Read current state of output port
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or al, 0x02 ; Set A20 bit
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call a20wait ; Wait for the keyboard controller to be ready
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mov al, 0xD1 ; Command to write output port
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out 0x64, al ; Send command to keyboard controller command port
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call a20wait ; Wait for the keyboard controller to be ready
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mov al, 0x02 ; New output port data with A20 enabled
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out 0x60, al ; Write new output port data
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call a20wait ; Wait for the keyboard controller to be ready
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sti ; Re-enable interrupts
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ret
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; Wait for keyboard controller to be ready
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a20wait:
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in al, 0x64 ; Read keyboard controller status port
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test al, 0x02 ; Check if input buffer is full
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jnz a20wait ; Wait until it's not full
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ret
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; Wait for keyboard controller to be ready
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a20wait:
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in al, 0x64 ; Read keyboard controller status port
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test al, 0x02 ; Check if input buffer is full
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jnz a20wait ; Wait until it's not full
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ret
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; Enter kernel space and jump to the kernel entry point
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JMP 0x1000:0x0000
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; Enter kernel space and jump to the kernel entry point
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JMP 0x1000:0x0000
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; Code to set up flat memory model for protected mode
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; This involves setting up the segment registers with selectors
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; that point to descriptors in the GDT that define a flat memory model
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; Code to set up flat memory model for protected mode
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; This involves setting up the segment registers with selectors
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; that point to descriptors in the GDT that define a flat memory model
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limit = 0x00CFFFFFh ; Define limit as a separate variable within gdt_struct
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; Define GDT structure
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gdt_struct:
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; Null descriptor (ignored)
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dq 0x0000000000000000h
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dq 0x0000000000000000h
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; Placeholder instruction to satisfy NASM
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dummy_instruction DB 0x90 ; NOP instruction as a placeholder
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; Code segment descriptor (flat memory model)
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; Base address 0, limit 0xffffffff (full 4GB)
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; Present, Readable, Accessed, Code segment, 4KB granularity
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; Flags: 0x9A (Bits explained below)
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dq 0x0000000000000000h
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dq 0x00CFFFFFFFh
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db 0x9A
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db 0xCF
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gdt_struct:
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; Data segment descriptor (flat memory model)
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; Similar structure to code segment descriptor with Data segment flag set
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dq 0x0000000000000000h
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dq 0x00CFFFFFFFh
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db 0x92
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db 0xCF
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base_addr equ 0x0000000
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; Jump to the kernel entry point
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JMP 0x1000:0x0000 ; Assuming the kernel is loaded at 0x1000:0x0000
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; Null descriptor (ignored)
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dd base_addr, 0 ; Both values are zero for a null descriptor
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; Code segment descriptor (flat memory model)
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dd base_addr, limit
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db 0x9A
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db 0xCF
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; Data segment descriptor (flat memory model)
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dd base_addr, limit
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db 0x92
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db 0xCF
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; Macro to copy boot sector data to a safe location
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mCopyBootSector:
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@@ -28,7 +28,4 @@ SECTIONS {
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bootloader_padding : {
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*(.bootloader_padding)
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}
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/* Set the bootloader size to 512 bytes */
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= 512;
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}
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