1 files changed, 34 insertions, 0 deletions
diff --git a/apps/bos/switch_to_pm.asm b/apps/bos/switch_to_pm.asm
new file mode 100644
index 00000000..aeef97c0
--- /dev/null
+++ b/apps/bos/switch_to_pm.asm
@@ -0,0 +1,34 @@
+[bits 16]
+; Switch to protected mode
+switch_to_pm:
+  cli                     ; We must switch of interrupts until we have
+                          ; set up the protected mode interrupt vector
+                          ; otherwise interrupts will run riot.
+
+  lgdt [gdt_descriptor]   ; Load our global descriptor table, which defines
+                          ; the protected mode segments (e.g. for code and data)
+
+  mov eax, cr0            ; To make the switch to protected mode, we set
+  or eax, 0x1             ; the first bit of CR0, a control register
+  mov cr0, eax
+
+  jmp CODE_SEG:init_pm    ; Make a far jump (i.e. to a new segment) to our 32-bit
+                          ; code. This also forces the CPU to flush its cache of
+                          ; prefetched and real-mode decoded instructions, which can
+                          ; cause problems.
+
+[bits 32]
+; Initialise registers and the stack once in PM.
+init_pm:
+
+  mov ax, DATA_SEG        ; Now in PM, our old segments are meaningless,
+  mov ds, ax              ; so we point our segment registers to the
+  mov ss, ax              ; data selector we defined in our GDT
+  mov es, ax
+  mov fs, ax
+  mov gs, ax
+
+  mov ebp, 0x90000        ; Update our stack position so it is right
+  mov esp, ebp            ; at the top of the free space.
+
+  call BEGIN_PM           ; Finally, call some well-known label