From f7ddd1a356f58298d703b11e862d3d6127e67080 Mon Sep 17 00:00:00 2001
From: Wojtek Kosior <kwojtus@protonmail.com>
Date: Sat, 28 Dec 2019 18:09:09 +0100
Subject: rewrite the stage 1 of bootloader in a more compact and (mostly)
 load-addr-independent way

---
 loader_stage1.S | 55 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 55 insertions(+)
 create mode 100644 loader_stage1.S

(limited to 'loader_stage1.S')

diff --git a/loader_stage1.S b/loader_stage1.S
new file mode 100644
index 0000000..adf2e12
--- /dev/null
+++ b/loader_stage1.S
@@ -0,0 +1,55 @@
+// armv7 mode
+ 
+// Entry point for the kernel.
+// r15 -> should begin execution at 0x8000.
+// r0 -> 0x00000000
+// r1 -> 0x00000C42
+// r2 -> 0x00000100 - start of ATAGS
+// preserve these registers as argument for kernel
+
+.global _boot
+_boot:
+        // Only let the first core execute
+        mrc p15, 0, r3, c0, c0, 5
+        and r3, r3, #3
+        cmp r3, #0
+        beq proceed
+	// this is a kind of blef - races can theoretically still occur
+	// when the main core overwrites this part of memory
+	wfe
+
+proceed:
+	// copy stage2 of the loader to address 0x4000
+
+	// first, load address of stage2_start to r3 (a PIC way)
+	adr r3, stage2_start
+
+	// load destination address for stage2 code to r4
+	mov r4, #0x4000
+
+	// load blob size to r5
+	mov r5, #(stage2_end - stage2_start)
+
+	// r6 is the counter - counts the bytes copied
+	mov r6, #0
+
+	// each word of the blob is loaded to r7 and stored
+	// from r7 to it's destination in a loop
+loop:
+	ldr r7, [r3, r6]
+	str r7, [r4, r6]
+	add r6, r6, #4
+	cmp r6, r5
+	blo loop
+	
+        // Initialize the stack
+	// _stack_top is defined in loader_stage1_linker.ld
+	ldr sp, =_stack_top
+	
+        // Call stage2 of the loader (branch to 0x4000)
+        bx r4
+
+.align 4
+stage2_start:	
+	.incbin "loader_stage2.img"
+stage2_end:
-- 
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