From 634673678a6e921e758f83df5f2d0acaffb9308f Mon Sep 17 00:00:00 2001
From: Eugen Wissner <belka@caraus.de>
Date: Sun, 6 Jul 2025 23:37:56 +0200
Subject: [PATCH] Map the kernel into the virtual memory

---
 kernel.ld |  3 +--
 kernel.s  | 42 ++++++++++++++++++++++++++----------------
 2 files changed, 27 insertions(+), 18 deletions(-)

diff --git a/kernel.ld b/kernel.ld
index ed1aa24..7f65958 100644
--- a/kernel.ld
+++ b/kernel.ld
@@ -4,6 +4,7 @@ ENTRY(boot)
 
 SECTIONS {
     . = 0x80200000;
+	__kernel_base = .;
 
     .text :{
         KEEP(*(.text.boot));
@@ -30,6 +31,4 @@ SECTIONS {
 
 	. = ALIGN(4096);
 	__free_ram = .;
-	. += 64 * 1024 * 1024; /* 64MB */
-	__free_ram_end = .;
 }
diff --git a/kernel.s b/kernel.s
index 933d21e..4d15247 100644
--- a/kernel.s
+++ b/kernel.s
@@ -34,24 +34,34 @@ kernel_main:
 	call device_tree
 
 	# Prepare the kernel root memory table.
+	la a0, __kernel_base
+	srli a0, a0, 22
+	andi a0, a0, 0x3ff # VPN[1].
+
+	# Multiply VPN[1] by 4 and add to the start address of the page table.
+	# This gives the physical address of the page table entry.
+	la t0, __free_ram
+	slli a0, a0, 2
+	add a0, t0, a0
+
+	# Calculate PPN[1] (10 most significant bits of the physical address
+	# and map it to 12 bits of the table page entry.
+	la a1, __kernel_base
+	srli a1, a1, 2
+	li t0, 0xff300000
+	and a1, a1, t0
+	ori a1, a1, 0b1111
+
+	sw a1, (a0)	# Save the page entry.
+
 	la a0, __free_ram
-	li a1, 8
-	call write_x
-
-	la t0, __free_ram
-	srli t0, t0, 10
-	li t1, 0xf0000000
-	or t0, t0, t1
-
-	la t1, __free_ram
-	sw t0, (t1)
-
-	la t0, __free_ram
-	ori t0, t0, 1
-
-	csrw satp, t0
-	sfence.vma zero, zero
+	srli a0, a0, 12
+	li t0, 0x80000000
+	or a0, a0, t0
 
+	sfence.vma
+	csrw satp, a0
+	sfence.vma
 
 	# Do nothing in a loop.
 .Lkernel_main: