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https://github.com/xcat2/xNBA.git
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Checking in because I don't want to lose this rather neat code for
running the decompresser in 16:16 protected mode using the real-mode stack. However, there's an even simpler way to do it...
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178
src/arch/i386/prefix/libprefix.S
Normal file
178
src/arch/i386/prefix/libprefix.S
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@ -0,0 +1,178 @@
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#define CR0_PE 1
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.arch i386
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.section ".prefix", "awx", @progbits
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/****************************************************************************
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* alloc_basemem (real-mode near call)
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*
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* Allocate space from base memory via the BIOS free base memory
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* counter at 40: 13
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*
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* Parameters:
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* %cx : Number of bytes to allocate
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* Returns:
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* %es : Segment address of newly allocated memory
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****************************************************************************
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*/
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.section ".prefix"
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.code16
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alloc_basemem:
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/* Preserve registers */
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pushw %cx
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pushw %ax
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/* %fs = 0x40, %ax = fbms */
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movw $40, %ax
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movw %ax, %fs
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/* Round up %cx to nearest kB, subtract from FBMS */
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addw $0x03ff, %cx
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andw $0xfc00, %cx
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shrw $10, %cx
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movw %fs:0x13, %ax
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subw %cx, %ax
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movw %ax, %fs:0x13
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/* Convert to segment address in %es */
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shlw $6, %ax
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movw %ax, %es
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/* Restore registers and return */
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popw %ax
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popw %cx
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ret
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.section ".prefix"
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.align 16
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gdt:
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gdt_limit: .word gdt_length - 1
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gdt_base: .long gdt
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.word 0 /* padding */
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cs16: /* 16 bit code segment, base at real-mode %cs:0000 */
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.equ CS16, cs16 - gdt
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.word 0xffff, 0
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.byte 0, 0x9b, 0, 0
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ss16: /* 16 bit stack segment, base at real-mode %ss:0000 */
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.equ SS16, ss16 - gdt
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.word 0xffff, 0
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.byte 0, 0x93, 0, 0
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flat_ds: /* 16 bit data segment, zero base, 4GB limit */
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.equ FLAT_DS, flat_ds - gdt
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.word 0xffff, 0
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.byte 0, 0x9f, 0xcf, 0
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gdt_end:
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.equ gdt_length, gdt_end - gdt
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.section ".prefix"
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.code16
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prot16_call:
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/* Install .data16 to top of base memory */
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movw %cs, %ax
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addw $_data16_load_offset_pgh, %ax
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movw %ax, %ds
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movw $_data16_size, %cx
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call alloc_basemem
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xorw %si, %si
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xorw %di, %di
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movw $_data16_progbits_size, %cx
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rep movsb /* or "call decompress16" */
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/* Install .code16 to top of base memory */
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movw %cs, %ax
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addw $_code16_load_offset_pgh, %ax
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movw %ax, %ds
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movw $_code16_size, %cx
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call alloc_basemem
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xorw %si, %si
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xorw %di, %di
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rep movsb /* or "call decompress16" */
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/* Push flags and real-mode segment registers */
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pushfl
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push %gs
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push %fs
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push %es
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push %ds
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push %ss
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push %cs
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/* Physical address of %cs:0000 to %ebx, of %ss:0000 to %eax */
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xorl %ebx, %ebx
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movw %cs, %bx
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shll $4, %ebx
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xorl %eax, %eax
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movw %ss, %ax
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shll $4, %eax
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/* Set up GDT and switch to protected mode */
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addl %ebx, %cs:gdt_base
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orl %ebx, %cs:(cs16+2)
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orl %eax, %cs:(ss16+2)
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cli
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data32 lgdt %cs:gdt
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movl %cr0, %eax
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orb $CR0_PE, %al
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movl %eax, %cr0
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data32 ljmp $CS16, $1f
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1: movw $SS16, %ax
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movw %ax, %ss
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movw $FLAT_DS, %ax
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movw %ax, %ds
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movw %ax, %es
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movw %ax, %fs
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movw %ax, %gs
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/* Install .text and .data to 2MB mark. Use 2MB to avoid
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* having to deal with A20.
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*/
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leal _text_load_offset(%ebx), %esi
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movl $( 2 * 1024 * 1024 ), %edi
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movl $_text_and_data_progbits_size, %ecx
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addr32 rep movsb /* or "call decompress16" */
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/* Restore real-mode segment limits */
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movw %ss, %ax
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movw %ax, %ds
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movw %ax, %es
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movw %ax, %fs
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movw %ax, %gs
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/* Return to real mode, restore segment registers and flags */
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pushw $1f
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movl %cr0, %eax
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andb $0!CR0_PE, %al
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movl %eax, %cr0
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lret /* used as equivalent of pop %cs */
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1: pop %ss
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pop %ds
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pop %es
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pop %fs
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pop %gs
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popfl
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/* Call init_gdt */
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pushw %cs
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pushw $1f
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pushw %es
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pushw $init_gdt
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lret /* lcall %es:init_gdt */
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1:
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ret
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