From 74d0e8c1f9527054a012aa9937c590537f9d3d5b Mon Sep 17 00:00:00 2001
From: Alejandro Sior <aho@sior.be>
Date: Fri, 20 May 2022 10:33:27 +0200
Subject: bios.call: clean and avoid hardcoding the gdt and idt when returning
 to long mode Do not make assumptions on the previous gdt and idt, as well as
 the state of the flags when operating.

---
 bios/bios.ha          |  6 +---
 drive/drive.ha        | 19 ++++++------
 rt/+x86_64/realcall.S | 81 +++++++++++++++++++++++++--------------------------
 rt/+x86_64/stage0.s   |  1 -
 4 files changed, 50 insertions(+), 57 deletions(-)

diff --git a/bios/bios.ha b/bios/bios.ha
index afae0f6..aa69f8a 100644
--- a/bios/bios.ha
+++ b/bios/bios.ha
@@ -7,11 +7,7 @@ export type state = struct {
 	@offset(12) edx: u32,
 	@offset(16) edi: u32,
 	@offset(20) esi: u32,
-	@offset(24) ds: u16,
-	@offset(26) es: u16,
-	@offset(28) ss: u16,
-	@offset(30) gs: u16,
-	@offset(32) fs: u16
+	@offset(24) es: u16,
 };
 
 // The set real mode registers
diff --git a/drive/drive.ha b/drive/drive.ha
index 42cef43..259e7f5 100644
--- a/drive/drive.ha
+++ b/drive/drive.ha
@@ -2,19 +2,20 @@ use bios;
 use bios::drive;
 
 fn lba_to_chs(lba: u16) (u16, u16, u16) = {
-	let temp = lba / bios::drive::drive_spt;
-	let sector = (lba % bios::drive::drive_spt) + 1;
-	let head = temp % bios::drive::drive_heads;
-	let cylinder = temp / bios::drive::drive_heads;
+	//let temp = lba / bios::drive::drive_spt;
+	//let sector = (lba % bios::drive::drive_spt) + 1;
+	//let head = temp % bios::drive::drive_heads;
+	//let cylinder = temp / bios::drive::drive_heads;
 	
-	return (cylinder, head, sector);
+	//return (cylinder, head, sector);
+	return (0, 0, 0);
 };
 
 export fn read(sector: u16, dest: uintptr) void = {
 	let chs = lba_to_chs(sector);
 
-	bios::regs.eax = 1 | 0x2 << 8;
-	bios::regs.ebx = ws;
-	bios::regs.ecx = chs.2 | chs.0 << 8;
-	bios::regs.edx = bios::drive::drive_no | chs.1 << 8;
+	//bios::regs.eax = 1 | 0x2 << 8;
+	//bios::regs.ebx = ws;
+	//bios::regs.ecx = chs.2 | chs.0 << 8;
+	//bios::regs.edx = bios::drive::drive_no | chs.1 << 8;
 };
diff --git a/rt/+x86_64/realcall.S b/rt/+x86_64/realcall.S
index 9e9afb4..ffbc0a9 100644
--- a/rt/+x86_64/realcall.S
+++ b/rt/+x86_64/realcall.S
@@ -14,18 +14,13 @@ redi:
 	.int 0x0
 resi:
 	.int 0x0
-rds:
-	.short 0x0
 res:
 	.short 0x0
-rss:
-	.short 0x0
-rgs:
-	.short 0x0
-rfs:
-	.short 0x0
-
 
+prev_idt:
+	.quad 0x0
+prev_gdt:
+	.quad 0x0
 
 # :real_call
 # This function is intended to be called from long mode
@@ -45,12 +40,25 @@ bios.call:
 	push %r13
 	push %r14
 	push %r15
+	pushf
+
+	# :Save code segment (and second push to prepare for far return)
+	mov %cs, %ax
+	push %ax
+	push %ax
+
+	# :Save data segment
+	mov %ds, %ax
+	push %ax
+	# :Save the interrupt number
 	push %di
 
 	cli
 
+	sidt (prev_idt)
+	sgdt (prev_gdt)
 
-	lgdt gdtr32
+	lgdt (gdtr32)
 	pushq $(gdt32_code - gdt32)
 	pushq $real_call_to_pmode_down
 	retfq
@@ -92,15 +100,12 @@ real_call_to_16bits_rmode_down:
 	mov %ax, %fs
 
 	lidt bios_idtr
-	sti
 
 	# :Self modifying code to call arbitrary interrupts
 	pop %ax
 	mov $real_call_int, %bx
 	mov %al, 1(%bx)
 
-	push %ds
-
 	# :Load registers
 	mov (reax), %eax
 	mov (rebx), %ebx
@@ -108,29 +113,22 @@ real_call_to_16bits_rmode_down:
 	mov (redx), %edx
 	mov (redi), %edi
 	mov (resi), %esi
-	mov %ds:(res), %es
-	mov %ds:(rss), %ss
-	mov %ds:(rgs), %gs
-	mov %ds:(rfs), %fs
-	mov %ds:(rds), %ds
-	xchg %bx, %bx
+	mov (res), %es
+
+	sti
 
 real_call_int:
 	int $0x0
-	
-	pop %ds
-	mov %ds:(rfs), %fs
-	mov %ds:(rgs), %gs
-	mov %ds:(rss), %ss
-	mov %ds:(res), %es
+
+	cli
+
+	mov %es, (res)
 	mov %eax, (reax)
 	mov %ebx, (rebx)
 	mov %ecx, (recx)
 	mov %edx, (redx)
 	mov %edi, (redi)
 	mov %esi, (resi)
-
-	cli
 	
 	# :Restore protected mode
 	mov %cr0, %eax
@@ -157,33 +155,32 @@ real_call_to_pmode_up:
 	or $1 << 31, %eax
 	mov %eax, %cr0
 
-	lgdt gdtr64
-	ljmp $(gdt64_code - gdt64), $real_call_to_longmode_up
+	lgdt (prev_gdt)
+
+	# :At this point %ds is latest in stack
+	pop %ax
+
+	# :At this point %cs is latest in stack
+	# Do a long jump
+	push $real_call_to_longmode_up
+	retf
 .code64
 real_call_to_longmode_up:
-	mov $(gdt64_data - gdt64), %ax
 	mov %ax, %fs
 	mov %ax, %gs
 	mov %ax, %ss
 	mov %ax, %es
 	mov %ax, %ds
 
+	# :Avoid doing this until the bootloader loads a 64 bits IDT
+	# XXX
+	#idt (prev_idt)
+
 real_call_end:
+	popf
 	pop %r15
 	pop %r14
 	pop %r13
 	pop %r12
 	pop %rbx
 	ret
-
-.globl testt
-testt:
-	mov $1, %al
-	mov $0x43, %bh
-	mov $0x6, %ah
-	mov $0, %ch
-	mov $0, %cl
-	mov $5, %dh
-	mov $5, %dl
-	int $0x10
-	ret
diff --git a/rt/+x86_64/stage0.s b/rt/+x86_64/stage0.s
index e17c06b..2961a10 100644
--- a/rt/+x86_64/stage0.s
+++ b/rt/+x86_64/stage0.s
@@ -89,5 +89,4 @@ bios_idtr:
 .org 510
 .word 0xaa55
 
-
 .include "rt/+x86_64/stage1.S"
-- 
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