ROM header

Every WonderSwan ROM contains a header. It is stored at the end - final sixteen bytes - of the ROM image.

ROM header

Parts of this header are validated or otherwise used by the console's boot ROM - they are marked in bold.

Header contents

Offset	Length	Contents
$0	5	Far jump instruction: 0xEA, offset, segment.
$5	1	Maintenance
$6	1	Developer/Publisher ID
$7	1	Color
$8	1	Game ID (binary-coded decimal)
$9	1	Game version? / Safe mode
$A	1	ROM size
$B	1	Save type/size
$C	1	Flags
$D	1	Mapper
$E	2	Checksum (sum of all ROM bytes excluding the checksum)

Maintenance ($5)

---

7  bit  0
---- ----
s... 0000
|    ||||
|    ++++- Must be zero; otherwise the console will prohibit execution.
|          Purpose unknown.
+--------- (Color) Splash bypass: 0 = Disable, 1 = Enable.
           Bypasses the custom boot splash stored in the console's
           internal EEPROM; presumably used for bricked console recovery.

Developer/Publisher ID ($6)

---

Known official developer/publisher IDs

ID	Publisher code	Publisher
$01	BAN	Bandai
$02	TAT	Taito
$03	TMY	Tomy
$04	KEX	Koei
$05	DTE	Data East
$06	AAE	Asmik Ace
$07	MDE	Media Entertainment
$08	NHB	Nichibutsu
$0A	CCJ	Coconuts Japan
$0B	SUM	Sammy
$0C	SUN	Sunsoft
$0D	PAW	Mebius
$0E	BPR	Banpresto
$10	JLC	Jaleco
$11	MGA	Imagineer
$12	KNM	Konami
$16	KBS	Kobunsha
$17	BTM	Bottom Up
$18	KGT	Kaga Tech
$19	SRV	Sunrise
$1A	CFT	Cyber Front
$1B	MGH	Megahouse
$1D	BEC	Interbec
$1E	NAP	Nihon Application
$1F	BVL	Bandai Visual
$20	ATN	Athena
$21	KDX	KID
$22	HAL	HAL Corporation
$23	YKE	Yuki Enterprise
$24	OMN	Omega Micott
$25	LAY	Layup
$26	KDK	Kadokawa Shoten
$27	SHL	Shall Luck
$28	SQR	Squaresoft
$2A	SCC	?
$2B	TMC	Tom Create
$2D	NMC	Namco
$2E	SES	?
$2F	HTR	?
$31	VGD	Vanguard
$32	MGT	Megatron
$33	WIZ	Wiz
$35	TAN	Tanita
$36	CPC	Capcom

Note that IDs $00 and $01 are used across a variety of commercial games. Homebrew is known to use arbitrary values in this field.

Color ($7)

---

7  bit  0
---- ----
???? ???c
        |
        +- 0 = Monochrome only, 1 = Supports Color mode.

Game version? / Safe mode ($9)

---

7  bit  0
---- ----
pvvv vvvv
|||| ||||
|+++ ++++- Game version?
+--------- Internal EEPROM write protect: 0 = Enable, 1 = Disable.
           This prohibits writing to the internal EEPROM for addresses
           0x60 and above (user settings, custom splash screen).
           Games are expected to enable write protection; however, the
           WonderWitch is factory configured with it disabled.

ROM size ($A)

---

Unofficial/inferred ROM size values are marked in italic.

ROM size values

ID	Size
$00	1 Mbit (128 KiB)
$01	2 Mbit (256 KiB)
$02	4 Mbit (512 KiB)
$03	8 Mbit (1 MiB)
$04	16 Mbit (2 MiB)
$05	24 Mbit (3 MiB)
$06	32 Mbit (4 MiB)
$07	48 Mbit (6 MiB)
$08	64 Mbit (8 MiB)
$09	128 Mbit (16 MiB)
$0A	256 Mbit (32 MiB)
$0B	512 Mbit (64 MiB)

Note that ROM sizes over 16 MiB are not supported by the Bandai 2001 mapper. The Bandai 2003 mapper is limited to 64 MiB.

Save type/size ($B)

---

Save type values

ID	Type	Size
$00	None
$01	SRAM	256 Kbit (32 KiB)
$02
$03		1 Mbit (128 KiB)
$04		2 Mbit (256 KiB)
$05		4 Mbit (512 KiB)
$10	EEPROM	1 Kbit (128 B)
$20		16 Kbit (2 KiB)
$50		8 Kbit (1 KiB)

Note that while ID $01 is commonly documented as 64 Kbit, all known cartridges using that value come with 256 Kbit SRAM chips. This requires further investigation.

Flags ($C)

---

7  bit  0
---- ----
???? ?SBv
      |||
      ||+- Starting screen orientation: 0 = Horizontal, 1 = Vertical.
      ||   Controls the splash screen's orientation.
      |+-- ROM bus width: 0 = 16-bit, 1 = 8-bit.
      +--- ROM access time: 0 = 3 CPU cycles, 1 = 1 CPU cycle.

Mapper ($D)

---

Known mapper types

ID	Mapper
$00	Bandai 2001 or KARNAK
$01	Bandai 2003