Add support for a "romdisk"

boot.asm

@@ -22,12 +22,26 @@ start_boot_process:
     mov r0, 0             ; booting from disk id 0
     jmp 0x00000800
 
+; load the boot sector of the romdisk and jump to it
+; inputs:
+; none
+; outputs:
+; none (returns if romdisk is not bootable)
+start_boot_process_from_romdisk:
     ; read sector 0 to 0x800
-    out r1, 0x00000800
-    out r0, 0
+    mov r0, 0
+    mov r1, 4
+    mov r2, 0x00000800
+    call read_sector
+
+    ; check for the bootable magic bytes
+    cmp [0x000009FC], 0x523C334C
+    ifnz ret
 
     ; now clean up and jump to the loaded binary
     call boot_cleanup
+    mov rsp, SYSTEM_STACK ; reset stack pointer
+    mov r0, 4             ; booting from disk id 4
     jmp 0x00000800
 
 ; clean up the system's state before jumping to the loaded binary

disk.asm

@@ -10,6 +10,9 @@ const TEMP_SECTOR_BUF: 0x01FFF808
 ; outputs:
 ; none
 read_sector:
+    cmp.8 r1, 4
+    ifz jmp read_romdisk_sector
+
     push r3
     push r4
 
@@ -23,6 +26,49 @@ read_sector:
     pop r3
     ret
 
+; read a sector from the romdisk into the specified memory buffer
+; inputs:
+; r0: sector number
+; r2: sector buffer (512 bytes)
+; outputs:
+; none
+read_romdisk_sector:
+    push r0
+    push r1
+    push r2
+
+    ; source pointer
+    mul r0, 512
+    add r0, romdisk_image
+
+    ; destination pointer
+    mov r1, r2
+
+    ; copy 512 bytes
+    mov r2, 512
+
+    call copy_memory_bytes
+
+    pop r2
+    pop r1
+    pop r0
+    ret
+
+; check if a RYFS image is included as a romdisk
+; inputs:
+; none
+; outputs:
+; Z flag: set if available, reset if not
+is_romdisk_available:
+    push r0
+
+    mov r0, romdisk_image
+    add r0, 514
+    cmp.16 [r0], 0x5952
+
+    pop r0
+    ret
+
 ; write a sector from the specified memory buffer
 ; inputs:
 ; r0: sector number
@@ -31,6 +77,9 @@ read_sector:
 ; outputs:
 ; none
 write_sector:
+    cmp.8 r1, 4
+    ifz ret
+
     push r3
     push r4
 

main.asm

@@ -161,6 +161,9 @@ event_loop:
     cmp r0, 0
     ifnz call start_boot_process
 
+    call is_romdisk_available
+    ifz call start_boot_process_from_romdisk
+
     jmp event_loop
 
 get_rom_version:
@@ -394,5 +397,9 @@ bottom_bar_patterns:
     data.32 0xFFFFFFFF
     data.32 0xFF674764
 
+romdisk_image:
+    #include_bin_optional "romdisk.img"
+romdisk_image_end:
+
     ; pad out to 512 KiB
     org.pad 0xF0080000