From: John Tsiombikas <nuclear@member.fsf.org>
Date: Wed, 16 Nov 2022 15:19:17 +0000 (+0200)
Subject: load stage2 high to leave the low RAM for the kernel
X-Git-Url: http://git.mutantstargoat.com/user/nuclear/?p=eightysix;a=commitdiff_plain;h=be7535b3af9d7ecc5581a7b3d325f5caec87b24f

load stage2 high to leave the low RAM for the kernel
---

diff --git a/.gitignore b/.gitignore
index 3576cff..77c7ebc 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,5 +3,6 @@
 *.swp
 *.img
 *.map
-dis
+dis1
+dis2
 *.log
diff --git a/Makefile b/Makefile
index 0bac632..8e95cce 100644
--- a/Makefile
+++ b/Makefile
@@ -26,4 +26,5 @@ debug: $(bin)
 
 .PHONY: disasm
 disasm: $(bin)
-	ndisasm -o 0x7c00 $< >dis
+	ndisasm -o 0x7c00 $< >dis1
+	ndisasm -o 0x80000 -e 512 $< >dis2
diff --git a/boot.ld b/boot.ld
index b607e11..99bc701 100644
--- a/boot.ld
+++ b/boot.ld
@@ -6,10 +6,13 @@ SECTIONS {
 	.bootsect : {
 		* (.bootsect);
 	}
+	_bootsect_end = .;
 
-	. = 0x7e00;
+	/* load high out of the way, to allow stage2 to load the kernel low */
+	. = 0x80000;
 	_stage2_start = .;
-	.text : {
+	_stage2_start_seg = _stage2_start >> 4;
+	.text : AT(_bootsect_end) {
 		* (.startup);
 		* (.text*);
 	}
@@ -19,6 +22,7 @@ SECTIONS {
 	}
 	.bss ALIGN(4) (NOLOAD): {
 		_bss_start = .;
+		_bss_start_off = _bss_start - _stage2_start;
 		* (.bss*);
 		* (COMMON);
 		. = ALIGN(4);
@@ -26,4 +30,5 @@ SECTIONS {
 	_bss_size = SIZEOF(.bss);
 	. = ALIGN(4);
 	_stage2_end = .;
+	_stage2_size = _stage2_end - _stage2_start;
 };
diff --git a/boot/boot.asm b/boot/boot.asm
index 5b82c2d..2b74db8 100644
--- 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]
diff --git a/boot/boot2.asm b/boot/boot2.asm
index 6837579..c90cef6 100644
--- a/boot/boot2.asm
+++ b/boot/boot2.asm
@@ -4,15 +4,25 @@
 	section .startup
 
 extern bootmain
+extern _stage2_start_seg
+extern _bss_start_off
+extern _bss_size
 
 global _start
 _start:
 	; TODO floppy off if necessary
-	; TODO zero .bss
+
+	; zero .bss
+	mov di, _bss_start_off
+	mov cx, _bss_size
+	shr cx, 1
+	rep stosw
+	xor ax, ax
+	mov es, ax
+
 	call bootmain
 
 hang:	hlt
 	jmp hang
 
-
 ; vi:set ts=8 sts=8 sw=8 ft=nasm: