| 1 | .globl entry, __switch_context, __exit_context, halt
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| 2 |
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| 3 | .text
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| 4 | .align 4
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| 5 |
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| 6 | /*
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| 7 | * Entry point
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| 8 | * We start execution from here.
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| 9 | * It is assumed that CPU is in 32-bit protected mode and
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| 10 | * all segments are 4GB and base zero (flat model).
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| 11 | */
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| 12 | entry:
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| 13 | /* Save boot context and switch to our main context.
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| 14 | * Main context is statically defined in C.
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| 15 | */
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| 16 | pushl %cs
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| 17 | call __switch_context
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| 18 |
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| 19 | /* We get here when the main context switches back to
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| 20 | * the boot context.
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| 21 | * Return to previous bootloader.
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| 22 | */
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| 23 | ret
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| 24 |
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| 25 | /*
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| 26 | * Switch execution context
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| 27 | * This saves registers, segments, and GDT in the stack, then
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| 28 | * switches the stack, and restores everything from the new stack.
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| 29 | * This function takes no argument. New stack pointer is
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| 30 | * taken from global variable __context, and old stack pointer
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| 31 | * is also saved to __context. This way we can just jump to
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| 32 | * this routine to get back to the original context.
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| 33 | *
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| 34 | * Call this routine with lcall or pushl %cs; call.
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| 35 | */
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| 36 | __switch_context:
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| 37 | /* Save everything in current stack */
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| 38 | pushfl /* 56 */
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| 39 | pushl %ds /* 52 */
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| 40 | pushl %es /* 48 */
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| 41 | pushl %fs /* 44 */
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| 42 | pushl %gs /* 40 */
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| 43 | pushal /* 8 */
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| 44 | subl $8, %esp
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| 45 | movw %ss, (%esp) /* 0 */
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| 46 | sgdt 2(%esp) /* 2 */
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| 47 |
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| 48 | #if 0
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| 49 | /* Swap %cs and %eip on the stack, so lret will work */
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| 50 | movl 60(%esp), %eax
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| 51 | xchgl %eax, 64(%esp)
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| 52 | movl %eax, 60(%esp)
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| 53 | #endif
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| 54 |
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| 55 | /* At this point we don't know if we are on flat segment
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| 56 | * or relocated. So compute the address offset from %eip.
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| 57 | * Assuming CS.base==DS.base==SS.base.
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| 58 | */
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| 59 | call 1f
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| 60 | 1: popl %ebx
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| 61 | subl $1b, %ebx
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| 62 |
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| 63 | /* Interrupts are not allowed... */
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| 64 | cli
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| 65 |
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| 66 | /* Current context pointer is our stack pointer */
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| 67 | movl %esp, %esi
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| 68 |
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| 69 | /* Normalize the ctx pointer */
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| 70 | subl %ebx, %esi
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| 71 |
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| 72 | /* Swap it with new value */
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| 73 | xchgl %esi, __context(%ebx)
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| 74 |
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| 75 | /* Adjust new ctx pointer for current address offset */
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| 76 | addl %ebx, %esi
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| 77 |
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| 78 | /* Load new %ss and %esp to temporary */
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| 79 | movzwl (%esi), %edx
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| 80 | movl 20(%esi), %eax
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| 81 |
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| 82 | /* Load new GDT */
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| 83 | lgdt 2(%esi)
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| 84 |
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| 85 | /* Load new stack segment with new GDT */
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| 86 | movl %edx, %ss
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| 87 |
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| 88 | /* Set new stack pointer, but we have to adjust it because
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| 89 | * pushal saves %esp value before pushal, and we want the value
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| 90 | * after pushal.
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| 91 | */
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| 92 | leal -32(%eax), %esp
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| 93 |
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| 94 | /* Load the rest from new stack */
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| 95 | popal
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| 96 | popl %gs
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| 97 | popl %fs
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| 98 | popl %es
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| 99 | popl %ds
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| 100 | popfl
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| 101 |
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| 102 | /* Finally, load new %cs and %eip */
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| 103 | lret
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| 104 |
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| 105 | __exit_context:
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| 106 | /* Get back to the original context */
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| 107 | pushl %cs
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| 108 | call __switch_context
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| 109 |
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| 110 | /* We get here if the other context attempt to switch to this
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| 111 | * dead context. This should not happen. */
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| 112 |
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| 113 | halt:
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| 114 | cli
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| 115 | hlt
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| 116 | jmp halt
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