| .globl entry, __switch_context, __exit_context, halt |
| |
| .text |
| .align 4 |
| |
| /* |
| * Entry point |
| * We start execution from here. |
| * It is assumed that CPU is in 32-bit protected mode and |
| * all segments are 4GB and base zero (flat model). |
| */ |
| entry: |
| /* Save boot context and switch to our main context. |
| * Main context is statically defined in C. |
| */ |
| pushl %cs |
| call __switch_context |
| |
| /* We get here when the main context switches back to |
| * the boot context. |
| * Return to previous bootloader. |
| */ |
| ret |
| |
| /* |
| * Switch execution context |
| * This saves registers, segments, and GDT in the stack, then |
| * switches the stack, and restores everything from the new stack. |
| * This function takes no argument. New stack pointer is |
| * taken from global variable __context, and old stack pointer |
| * is also saved to __context. This way we can just jump to |
| * this routine to get back to the original context. |
| * |
| * Call this routine with lcall or pushl %cs; call. |
| */ |
| __switch_context: |
| /* Save everything in current stack */ |
| pushfl /* 56 */ |
| pushl %ds /* 52 */ |
| pushl %es /* 48 */ |
| pushl %fs /* 44 */ |
| pushl %gs /* 40 */ |
| pushal /* 8 */ |
| subl $8, %esp |
| movw %ss, (%esp) /* 0 */ |
| sgdt 2(%esp) /* 2 */ |
| |
| #if 0 |
| /* Swap %cs and %eip on the stack, so lret will work */ |
| movl 60(%esp), %eax |
| xchgl %eax, 64(%esp) |
| movl %eax, 60(%esp) |
| #endif |
| |
| /* At this point we don't know if we are on flat segment |
| * or relocated. So compute the address offset from %eip. |
| * Assuming CS.base==DS.base==SS.base. |
| */ |
| call 1f |
| 1: popl %ebx |
| subl $1b, %ebx |
| |
| /* Interrupts are not allowed... */ |
| cli |
| |
| /* Current context pointer is our stack pointer */ |
| movl %esp, %esi |
| |
| /* Normalize the ctx pointer */ |
| subl %ebx, %esi |
| |
| /* Swap it with new value */ |
| xchgl %esi, __context(%ebx) |
| |
| /* Adjust new ctx pointer for current address offset */ |
| addl %ebx, %esi |
| |
| /* Load new %ss and %esp to temporary */ |
| movzwl (%esi), %edx |
| movl 20(%esi), %eax |
| |
| /* Load new GDT */ |
| lgdt 2(%esi) |
| |
| /* Load new stack segment with new GDT */ |
| movl %edx, %ss |
| |
| /* Set new stack pointer, but we have to adjust it because |
| * pushal saves %esp value before pushal, and we want the value |
| * after pushal. |
| */ |
| leal -32(%eax), %esp |
| |
| /* Load the rest from new stack */ |
| popal |
| popl %gs |
| popl %fs |
| popl %es |
| popl %ds |
| popfl |
| |
| /* Finally, load new %cs and %eip */ |
| lret |
| |
| __exit_context: |
| /* Get back to the original context */ |
| pushl %cs |
| call __switch_context |
| |
| /* We get here if the other context attempt to switch to this |
| * dead context. This should not happen. */ |
| |
| halt: |
| cli |
| hlt |
| jmp halt |
