From d2cad77eb97eb671cf07c766ad9a65d9f0c1ee61 Mon Sep 17 00:00:00 2001
From: Gwilym Kuiper <gw@ilym.me>
Date: Sat, 26 Jun 2021 22:25:27 +0100
Subject: [PATCH] Add multiboot support

---
 agb/build.rs  |   1 +
 agb/crt0.s    |  46 ++++++++++++++++++++++
 agb/gba_mb.ld | 107 ++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 154 insertions(+)
 create mode 100644 agb/gba_mb.ld

diff --git a/agb/build.rs b/agb/build.rs
index 071b4389..be2401d1 100644
--- a/agb/build.rs
+++ b/agb/build.rs
@@ -2,6 +2,7 @@ use agb_image_converter::{convert_image, Colour, ImageConverterConfig, TileSize}
 
 fn main() {
     println!("cargo:rerun-if-changed=crt0.s");
+    println!("cargo:rerun-if-changed=gba_mb.ld");
     println!("cargo:rerun-if-changed=interrupt_simple.s");
     println!("cargo:rerun-if-changed=gfx/test_logo.png");
 
diff --git a/agb/crt0.s b/agb/crt0.s
index f904d2cd..47de3b86 100644
--- a/agb/crt0.s
+++ b/agb/crt0.s
@@ -6,6 +6,48 @@ __start:
     @ Filled in by gbafix
     .fill 188, 1, 0
 
+    @ multiboot launch point
+b .Initialise_mb
+
+    .byte 0 @ boot mode, BIOS overwrites this value
+    .byte 0 @ slave ID number
+    .fill 26, 1, 0 @ unused?
+    .word 0 @ joybus entrypoint
+
+.Initialise_mb:
+    swi 0x00250000
+
+    @ Set interrupt handler
+    ldr r0, =InterruptHandlerSimple
+    ldr r1, =0x03007FFC
+    str r0, [r1]
+
+    @ copies iwram section in ewram to iwram
+    ldr r0, =__iwram_rom_start              @ load memory address storing start of data for iwram in rom
+    ldr r1, =__iwram_data_start             @ load memory address storing location of iwram in ram
+    ldr r2, =__iwram_rom_length_halfwords   @ load number of 16 bit values to copy
+    swi 0x000B0000                          @ call interrupt CpuSet.
+                                            @   r0: source
+                                            @   r1: destination
+                                            @   r2: length + size information
+                                            @
+                                            @ see: https://mgba-emu.github.io/gbatek/#swi-0bh-gbands7nds9dsi7dsi9---cpuset
+    
+    @ everything is in ewram so it is already there, no need to copy that bit
+
+    @ put zero in both r0 and r1
+    @ This corresponds to zero for argc and argv (which would technically be required for a c runtime)
+    ldr r0, =0
+    mov r1, r0
+
+    @ load main and branch
+    ldr r2, =main
+    bx  r2
+
+    @ loop if we end up here
+1:
+    b 1b
+
 .Initialise:
     @ Set interrupt handler
     ldr r0, =InterruptHandlerSimple
@@ -39,6 +81,10 @@ __start:
     @ load main and branch
     ldr r2, =main
     bx  r2
+
+    @ loop if we end up here
+1:
+    b 1b
 .pool
 
 .include "interrupt_simple.s"
diff --git a/agb/gba_mb.ld b/agb/gba_mb.ld
new file mode 100644
index 00000000..df1a784a
--- /dev/null
+++ b/agb/gba_mb.ld
@@ -0,0 +1,107 @@
+OUTPUT_FORMAT("elf32-littlearm", "elf32-bigarm", "elf32-littlearm")
+OUTPUT_ARCH(arm)
+
+ENTRY(__start)
+
+MEMORY {
+    ewram (w!x) : ORIGIN = 0x02000000, LENGTH = 256K
+    iwram (w!x) : ORIGIN = 0x03000000, LENGTH = 32K
+}
+
+__text_start = ORIGIN(ewram);
+
+SECTIONS {
+    . = __text_start;
+
+    .crt0 : {
+		KEEP (crt0.o(.text));
+		. = ALIGN(4);
+    } > ewram
+
+    .text : {
+        *(.text .text*);
+        . = ALIGN(4);
+    } > ewram
+    __text_end = .;
+
+    .rodata : {
+        *(.rodata .rodata.*);
+        . = ALIGN(4);
+    } > ewram
+
+    __iwram_rom_start = .;
+    .iwram : {
+        __iwram_data_start = ABSOLUTE(.);
+
+        *(.iwram .iwram.*);
+        . = ALIGN(4);
+
+        *(.text_iwram .text_iwram.*);
+        . = ALIGN(4);
+
+        __iwram_data_end = ABSOLUTE(.);
+    } > iwram AT>ewram
+
+    . = __iwram_rom_start + (__iwram_data_end - __iwram_data_start);
+
+    __ewram_rom_start = .;
+    .ewram : {
+        __ewram_data_start = ABSOLUTE(.);
+
+        *(.ewram .ewram.*);
+        . = ALIGN(4);
+
+        *(.data .data.*);
+        . = ALIGN(4);
+
+        __ewram_data_end = ABSOLUTE(.);
+    } > ewram AT>ewram
+    
+    .bss : {
+        *(.bss .bss.*);
+        . = ALIGN(4);
+    } > iwram
+
+    __iwram_rom_length_bytes = __iwram_data_end - __iwram_data_start;
+    __iwram_rom_length_halfwords = (__iwram_rom_length_bytes + 1) / 2;
+
+    __ewram_rom_length_bytes = __ewram_data_end - __ewram_data_start;
+    __ewram_rom_length_halfwords = (__ewram_rom_length_bytes + 1) / 2;
+
+        /* debugging sections */
+    /* Stabs */
+    .stab            0 : { *(.stab) }
+    .stabstr         0 : { *(.stabstr) }
+    .stab.excl       0 : { *(.stab.excl) }
+    .stab.exclstr    0 : { *(.stab.exclstr) }
+    .stab.index      0 : { *(.stab.index) }
+    .stab.indexstr   0 : { *(.stab.indexstr) }
+    .comment         0 : { *(.comment) }
+    /* DWARF 1 */
+    .debug           0 : { *(.debug) }
+    .line            0 : { *(.line) }
+    /* GNU DWARF 1 extensions */
+    .debug_srcinfo   0 : { *(.debug_srcinfo) }
+    .debug_sfnames   0 : { *(.debug_sfnames) }
+    /* DWARF 1.1 and DWARF 2 */
+    .debug_aranges   0 : { *(.debug_aranges) }
+    .debug_pubnames  0 : { *(.debug_pubnames) }
+    /* DWARF 2 */
+    .debug_info      0 : { *(.debug_info) }
+    .debug_abbrev    0 : { *(.debug_abbrev) }
+    .debug_line      0 : { *(.debug_line) }
+   	.debug_frame     0 : { *(.debug_frame) }
+    .debug_str       0 : { *(.debug_str) }
+    .debug_loc       0 : { *(.debug_loc) }
+    .debug_macinfo   0 : { *(.debug_macinfo) }
+    /* SGI/MIPS DWARF 2 extensions */
+    .debug_weaknames 0 : { *(.debug_weaknames) }
+    .debug_funcnames 0 : { *(.debug_funcnames) }
+    .debug_typenames 0 : { *(.debug_typenames) }
+    .debug_varnames  0 : { *(.debug_varnames) }
+
+    .debug_ranges   0 : { *(.debug_ranges) }
+
+    /* discard anything not already mentioned */
+    /DISCARD/ : { *(*) }
+}
\ No newline at end of file