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-rw-r--r--qemu/roms/openbios/arch/amd64/switch.S116
1 files changed, 116 insertions, 0 deletions
diff --git a/qemu/roms/openbios/arch/amd64/switch.S b/qemu/roms/openbios/arch/amd64/switch.S
new file mode 100644
index 000000000..66668150d
--- /dev/null
+++ b/qemu/roms/openbios/arch/amd64/switch.S
@@ -0,0 +1,116 @@
+ .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