load stage2 high to leave the low RAM for the kernel

[eightysix] / boot / boot.asm

diff --git a/boot/boot.asm b/boot/boot.asm
index 5b82c2d..2b74db8 100644 (file)
--- a/boot/boot.asm
+++ b/boot/boot.asm
@@ -3,10 +3,9 @@
        bits 16
        section .bootsect
 
-extern _stage2_start
-extern _stage2_end
+extern _stage2_start_seg
+extern _stage2_size
 
-stacktop       equ 7b00h
 boot_driveno   equ 7b00h
 num_read_tries equ 7b06h       ; 2 bytes
 sect_pending   equ 7b08h       ; 2 bytes
@@ -58,12 +57,12 @@ start:
        xor ax, ax
        mov ds, ax
        mov es, ax
-       mov ss, ax
-       mov gs, ax
-       mov fs, ax
        jmp 00:.setcs
 .setcs:
-       mov sp, stacktop
+       ; put the stack high
+       mov ax, 0x7f00
+       mov ss, ax
+       xor sp, sp
        mov [boot_driveno], dl
 
        ; query sectors per track
@@ -84,20 +83,15 @@ start:
        mov word [num_heads], 2
 .querydone:
 
-; load the rest of the code at 7e00h
-       mov ax, _stage2_end
-       sub ax, _stage2_start
+       ; load the rest of the code high
+       mov ax, _stage2_size
        add ax, 511
        mov cl, 9
        shr ax, cl
        inc ax
        mov [sect_pending], ax
-       mov ax, _stage2_start
-       shr ax, 1
-       shr ax, 1
-       shr ax, 1
-       shr ax, 1
-       mov es, ax      ; destination segment 7e0h to allow loading up to 64k
+       mov ax, _stage2_start_seg
+       mov es, ax      ; destination segment
        mov word [destptr], 0
        mov word [start_sect], 1        ; start from sector 1 to skip boot sector
        mov word [cur_cyl], 0
@@ -170,10 +164,15 @@ start:
        add [destptr], ax
        jnz .rdloop
 
-       ; loaded sucessfully, reset es back to 0 and jump
-.done: xor ax, ax
+       ; loaded sucessfully, load segment registers and jump
+.done: mov ax, _stage2_start_seg
+       mov ds, ax
        mov es, ax
-       jmp _stage2_start
+       push ax
+       xor ax, ax
+       push ax
+       retf
+
 
 .fail: add sp, 2       ; clear num_sect off the stack
        dec word [num_read_tries]