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actually set cs to 16-bit segment for a bit in-case we want to run some pm code

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Elaina Claus
2025-09-05 15:26:24 -04:00
parent 5fac10f02c
commit b9b3e92632
1 changed files with 14 additions and 12 deletions
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26
src/stage2/stage2.nasm
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@@ -192,21 +192,22 @@ hcf:
; ##############################
ALIGN 4, db 0x90
EnterUnrealMode:
__BOCHS_MAGIC_DEBUG
__CDECL16_ENTRY
cli ; no interrupts
.func:
lgdt [((__STAGE2_SEGMENT << 4) + unreal_gdt_info)] ; load unreal gdt
mov eax, cr0
or al,1 ; set pmode bit
or eax, 1 ; set pmode bit
mov cr0, eax ; switch to pmode
jmp short $+2 ; i-cache flush
;jmp short $+2 ; i-cache flush
; set cs to a pm code segment (0x8) w/ the following
;jmp far 0x0008:EnterUnrealMode.set_segs
db 0xEA ; jmp far imm16:imm16
dw EnterUnrealMode.set_segs ; error_far_ptr
dw 0x0008 ; error_far_seg
jmp 0x0008:EnterUnrealMode.set_segs
;db 0xEA ; jmp far imm16:imm16
;dw EnterUnrealMode.set_segs ; error_far_ptr
;dw 0x0008 ; error_far_seg
.set_segs:
mov ax, 0x10 ; select descriptor 2
mov ds, ax ; 10h = 0001_0000b
@@ -216,13 +217,14 @@ EnterUnrealMode:
mov fs, ax
mov gs, ax ; extra segments to big data as well
.pm_start:
; code here is running in protected mode
; code here is running in protected mode w/ descriptor 0x8
; insert any PM test code needed here
.pm_end:
mov eax, cr0
and al,0xFE ; toggle bit 1 of cr0
and eax, ~1 ; toggle bit 1 of cr0
mov cr0, eax ; back to realmode
jmp short $+2 ; i-cache flush
jmp 0x0000:EnterUnrealMode.endp
;jmp short $+2 ; i-cache flush
.endp:
sti ; re-enable interupts
__CDECL16_EXIT
@@ -302,14 +304,14 @@ unreal_gdt_start:
; entry 0 (null descriptor)
dq 0 ; first entry is null
; entry 1 (16bit code 64KiB limit)
; entry 1 (0x8) (16bit code 64KiB limit)
dd 0x0000FFFF ; Base Address(15:0) 31:16, Segment Limit(15:0) 15:0
db 0x00 ; Base Address 23:16
db 1001_1010b ; Access Byte: Present, ring0, S = 1, executable (1), non-conforming, readable, Accessed
db 0000_0000b ; Flags: GR = 4KiB, attr = <DB/L/Avl>, Granularity = 4KiB & 16:19 of limit
db 0x00 ; Base Address 31:24
; entry 2 (16bit data segment with 4 GiB flat mapping)
; entry 2 (0x10) (16bit data segment with 4 GiB flat mapping)
dd 0x0000FFFF ; Base Address(15:0) 31:16, Segment Limit(15:0) 15:0
db 0x00 ; Base Address(23:16)
db 1001_0010b ; Access Byte: Present, ring0, S = 1, data (0), non-confirming, writable, present