wip starting item support

We'll start with just the SW Sea chart, but this should be expanded
in the future to eventually support any arbitrary item(s) the player
chooses.

base/code/main.asm

@@ -83,6 +83,21 @@
                 mov r1, 0x7d
                 ldr r0, [r0]
                 pop pc
+
+            .thumb
+            @set_starting_items:
+                push lr
+
+                push r1 ; save r1, as it's used later on in the original code
+                bl set_starting_items
+                pop r1 ; restore r1
+
+                ; These three instructions are the original instructions that the `bl` to this current
+                ; function replaced. They are needed to be able to return to the original code.
+                mov r0, 0x3C
+                mul r0, r1
+
+                pop pc
         .pool
         .endarea
 
@@ -129,6 +144,16 @@
         .pool
         .endarea
 
+    .thumb
+    .org 0x20ad20e
+        .area 0x6, 0x00
+            ; hook into the function that sets the starting item flags
+            bl @set_starting_items
+            ; original instruction, do not change
+            b 0x20ad216
+        .pool
+        .endarea
+
     .thumb
     .org 0x20ae1c2
         .area 0x6, 0x00
@@ -439,4 +464,5 @@
         .importobj "code/spawn_custom_freestanding_item.o"
         .importobj "code/custom_salvage_item.o"
         .importobj "code/extend_give_item_function.o"
+        .importobj "code/set_starting_items.o"
 .close

base/code/set_starting_items.c

@@ -0,0 +1,25 @@
+#include <stdbool.h>
+#include <stdint.h>
+
+typedef struct {
+  uint8_t address;
+  uint8_t bit;
+} ItemFlag;
+
+// define array of ints
+ItemFlag starting_items[] = {
+    // Oshus sword
+    // {0x0, 0x1}, // TODO: enable this to support "Start with sword setting"
+
+    // SW Sea Chart
+    {0x8, 0x2}, // TODO: make this dynamic when the starting island is randomized
+};
+
+__attribute__((target("thumb"))) void set_starting_items(void) {
+  uint8_t *base_address = (uint8_t *)(0x21BA604);
+
+  for (int i = 0; i < sizeof(starting_items) / sizeof(ItemFlag); i++) {
+    uint8_t *flag = (uint8_t *)(starting_items[i].address + base_address);
+    *flag |= starting_items[i].bit;
+  }
+}