experimenting with a20 gate

3 files changed, 91 insertions, 10 deletions

diff --git a/boot/check_a20_16.asm b/boot/check_a20_16.asm
new file mode 100644
index 0000000..43da6e3
--- /dev/null
+++ b/boot/check_a20_16.asm
@@ -0,0 +1,61 @@
+; The following code is public domain licensed
+ 
+[bits 16]
+ 
+; Function: check_a20
+;
+; Purpose: to check the status of the a20 line in a completely self-contained state-preserving way.
+;          The function can be modified as necessary by removing push's at the beginning and their
+;          respective pop's at the end if complete self-containment is not required.
+;
+; Returns: 0 in ax if the a20 line is disabled (memory wraps around)
+;          1 in ax if the a20 line is enabled (memory does not wrap around)
+ 
+check_a20:
+    pushf
+    push ds
+    push es
+    push di
+    push si
+ 
+    cli
+ 
+    xor ax, ax ; ax = 0
+    mov es, ax
+ 
+    not ax ; ax = 0xFFFF
+    mov ds, ax
+ 
+    mov di, 0x0500
+    mov si, 0x0510
+ 
+    mov al, byte [es:di]
+    push ax
+ 
+    mov al, byte [ds:si]
+    push ax
+ 
+    mov byte [es:di], 0x00
+    mov byte [ds:si], 0xFF
+ 
+    cmp byte [es:di], 0xFF
+ 
+    pop ax
+    mov byte [ds:si], al
+ 
+    pop ax
+    mov byte [es:di], al
+ 
+    mov ax, 0
+    je check_a20__exit
+ 
+    mov ax, 1
+ 
+check_a20__exit:
+    pop si
+    pop di
+    pop es
+    pop ds
+    popf
+ 
+    ret
diff --git a/boot/mbr.asm b/boot/mbr.asm
index 26d13bf..4dd82d1 100644
--- a/boot/mbr.asm
+++ b/boot/mbr.asm
@@ -56,20 +56,23 @@ jmp boot_16 ;start boot process
 BOOT_DRIVE:
     db 0xff
 STR_VERSION:
-    db "Fool Boot Loader v0.3.1~",0
+    db "v0.3.1~",0
 VESA_CHECK1:
-    db " VESA: Get Info.",0
+    db " V1.",0
 VESA_CHECK2:
-    db " VESA: Get Mode Info.",0
+    db " V2",0
 VESA_CHECK3:
-    db " VESA: Set Mode.",0
+    db " V2",0
+CHECK_A20:
+    db " A20",0
 
 ;lets put our temporary GDT (Global Descriptor Table) here
 ;kernel should move this away
 %include "boot/GDT.asm"
 
-;include 16-bit real mode routines (print_string, disk_load, vesa_setup)
+;include 16-bit real mode routines (print_string, disk_load, vesa_setup,check_a20)
 %include "boot/disk_load_16.asm"
+%include "boot/check_a20_16.asm"
 %include "boot/print_string_16.asm"
 %include "boot/vesa_setup_16.asm"
 
@@ -112,6 +115,15 @@ boot_16:
     call BiosGetMemoryMap    ; this will also put the number of entries 
 			    ; of the memory map at MEMMAP_SIZE_OFFSET
     
+a20check:
+
+    mov bx, STR_VERSION
+    call print_string
+
+    call check_a20
+    cmp ax,0
+    je a20check ;hang if a20 is disabled!
+
     ;VESA:  also setup vesa stuff before entering 32 bit protected mode
     call VesaSetup
 
@@ -127,9 +139,11 @@ boot_32_pm:
     ;enable A20
     ;http://www.brokenthorn.com/Resources/OSDev9.html
     ;Method 3.1: Enables A20 through keyboard controller
+    ;Not all keyboard controllers support this
     ;todo: check if this has any effect at all !?!?
-    mov al, 0xdd	; command 0xdd: enable a20
-    out 0x64, al	; send command to controller
+   ;mov al, 0xdd	; command 0xdd: enable a20
+   ;mov al, 0xdf	; command 0xdf: disable a20
+   ;out 0x64, al	; send command to controller
 
     ;pic setup
     call pic_setup
diff --git a/kernel/kernel.c b/kernel/kernel.c
index 3296b37..7d9b848 100644
--- a/kernel/kernel.c
+++ b/kernel/kernel.c
@@ -102,12 +102,18 @@ void kernel_main()
 
     //init shell
     shell_init();
+    /*
 
-/*
    log(FOOLOS_MODULE_NAME,FOOLOS_LOG_INFO,"test mem");
-   uint8_t *memtest=0x1fffffff;
+
+   // test a20 disabled / wrap
+   uint8_t *memtest=0x0;
+   uint8_t *memtest2=0b10000000000000000000;
+
    *memtest=0xaf;
-   log(FOOLOS_MODULE_NAME,FOOLOS_LOG_INFO,"test mem %x",*memtest);
+   log(FOOLOS_MODULE_NAME,FOOLOS_LOG_INFO,"test mem %x",*(memtest2-1));
+   log(FOOLOS_MODULE_NAME,FOOLOS_LOG_INFO,"test mem %x",*memtest2);
+   log(FOOLOS_MODULE_NAME,FOOLOS_LOG_INFO,"test mem %x",*(memtest2+1));
    */