120 lines
3.5 KiB
ArmAsm
120 lines
3.5 KiB
ArmAsm
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// vim: et:sw=12:ts=12:sts=12
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.global _start
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.text
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.code16
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.set SS_CODE16, 0x08
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.set SS_DATA16, 0x10
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.set SS_CODE32, 0x18
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.set SS_DATA32, 0x20
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// _start: entry point
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_start: cli
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cld
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mov %sp, %bp
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xor %ax, %ax
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mov %ax, %ds
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mov %ax, %es
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push $msg_start
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call dbgmsg
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add $2, %sp
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// initialize our own BSS section
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mov $_bss_start, %di
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mov $_bss_end, %cx
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sub %di, %cx
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xor %al, %al
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rep stosb
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// a20_enable: Allow use of 'high' (>1Mb) memory
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a20_enable: // Of all the ways to toggle A20, we only try the Fast A20 Gate.
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// This is known to cause problems on some ancient systems,
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// but as our bootloader exclusively runs on 64-bit machines,
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// we should not run into any of those systems.
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// Modern machines apparently don't even have the A20 line anymore.
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push $msg_a20
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call dbgmsg
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add $2, %sp
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inb $0x92, %al
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or $2, %al
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outb %al, $0x92
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// prot_enter: Set up GDT, switch into 32-bit protected mode.
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prot_enter:
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push $msg_prot
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call dbgmsg
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add $2, %sp
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lgdt GDT_PTR
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// Set Protection Enable (PE) bit
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mov %cr0, %eax
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or $1, %al
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mov %eax, %cr0
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ljmp $SS_CODE32, $prot_trampo
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.code32
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prot_trampo:
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mov $SS_DATA32, %eax
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mov %eax, %ds
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mov %eax, %es
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mov %eax, %fs
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mov %eax, %gs
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mov %eax, %ss
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mov 'Y', %al
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outb %al, $0xE9
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// TODO load proper stack pointer
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mov $0x90000, %ebp
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mov %ebp, %esp
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jmp main
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.code16
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dbgmsg: push %bp
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mov %sp, %bp
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push %ax
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push %si
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mov 4(%bp), %si
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1: lodsb
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test %al, %al
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jz 2f
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outb %al, $0xE9
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jmp 1b
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2: pop %si
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pop %ax
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pop %bp
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ret
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.data
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.global GDT
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GDT: // entry 0: null descriptor
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.space 8, 0
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// entry 1: 16-bit code segment
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.byte 0xFF, 0xFF, 0, 0, 0, 0b10011010, 0x8F, 0
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// entry 2: 16-bit data segment
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.byte 0xFF, 0xFF, 0, 0, 0, 0b10010010, 0x8F, 0
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// entry 3: 32-bit code segment
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.byte 0xFF, 0xFF, 0, 0, 0, 0b10011010, 0xCF, 0
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// entry 4: 32-bit data segment
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.byte 0xFF, 0xFF, 0, 0, 0, 0b10010010, 0xCF, 0
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// TODO: 32-bit TSS
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.set GDT_SIZE, . - GDT
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GDT_PTR: .word GDT_SIZE - 1
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.quad GDT
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msg_start: .asciz "Netboot via fernlader v2 ...\r\n"
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msg_a20: .asciz " * Enabling A20 Gate\r\n"
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msg_prot: .asciz " * Protected Mode\r\n"
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