switched to 16bit mode, but int 10h hangs

diff --git a/efitest.asm b/efitest.asm
index bae6aa2..ae804dd 100644 (file)
--- a/efitest.asm
+++ b/efitest.asm
@@ -116,7 +116,7 @@ mmapbuf: times 4096 db 0
 str_hello db 'hello!',13,10,0
 
        align 4
-gdtlim dw 23
+gdtlim dw 31
 gdtbase        dq gdt
 
        align 8
@@ -128,7 +128,9 @@ gdt:        ; 0: null segment
        ; 2: data - base:0, lim:4g, G:4k, 32bit, avl, pres|app, dpl:0, type:data/rw
        dd 0x0000ffff
        dd 0x00cf9200
-
+       ; 3: code16
+       dd 0x0000ffff
+       dd 0x00009a00
 
        bits 32
 start32:
@@ -151,13 +153,8 @@ start32:
        and eax, 0fffffeffh
        wrmsr
 
-       mov edi, 0a0000h
-       mov ecx, 16000
-       mov eax, 00ff0000h
-       rep stosd
-
-       mov esi, str_hello
-       call ser_putstr
+       ; load 16bit code segment and jump to 16bit code
+       jmp 0x18:start16
 
 .hang: hlt
        jmp .hang
@@ -186,4 +183,44 @@ ser_putstr:
 
 
        align 4
+       ; real mode IDTR pseudo-descriptor pointing to the IVT at addr 0
+       dw 0
+rmidt: dw 3ffh
+       dd 0
+
+       bits 16
+start16:
+       ; disable protection
+       mov eax, cr0
+       and eax, 0fffffffeh
+       mov cr0, eax
+       ; load cs <- 0
+       jmp 0:.loadcs0
+.loadcs0:
+       ; zero data segments
+       xor ax, ax
+       mov ds, ax
+       mov es, ax
+       mov fs, ax
+       mov gs, ax
+       ; move stack to the top of 640k
+       mov ax, 9000h
+       mov ss, ax
+       xor sp, sp
+
+       ; run 16bit video bios test
+       mov ax, 13h
+       ;int 10h
+
+       mov ax, 0a000h
+       mov es, ax
+       xor di, di
+       mov cx, 32000
+       mov ax, 6767h
+       rep stosw
+
+.hang: hlt
+       jmp .hang
+
+       align 4
 end: