BIOS

Table of Contents

• Firmware Subroutines
  • Flash Memory Write
  • Flash Memory Verify
  • Flash Memory Read
  • Clock Update
  • Mode Change
• System Variables

Built-in OS and Firmware located within ROM.

Firmware Subroutines

The VMU BIOS contains a series of built-in subroutines, which are exposed as public ROM entry-points which the user application may invoke to perform various system-related tasks.

To call one of these subroutines, use a NOT1 EXT, 0 instruction followed IMMEDIATELY by a JMPF instruction to the entry address. This swaps the CPU from executing out of flash to executing out of ROM and continues execution at the proper routine. Each routine has an advertised exit address where it will return control back to the user-application at a fixed address, as well as fixed input/output variables which will be passed through RAM bank 1.

Flash Memory Write

Writes a single contiguous page of data (128 bytes) to Flash, starting at the specified address.

Entry Address
ORG 0x100

Arguments

Address	Name	Function
0x7D	fmadd_bank	Flash Bank (must be 0)
0x7E	fmadd_h	Starting address upper byte
0x7F	fmadd_l	Starting address lower byte
0x80-0xFF	input data	Data to be written to flash

Warning

The fmadd address must be 128-byte page-aligned (bits 0-6 of the lower address reset to 0). Only a single full page can be written to at a time.

The routine requires flash bank be set to 0 or "normal operation is not assured."

Make sure to call the routine with STAD set to 0x00, or part of the screen will be overwritten.

Return Values

ACC Value	Meaning
0x0	Normal end
0xFF	Abnormal end

Clobbered Registers

• ACC
• B
• C
• TRL
• TRH

Flash Memory Verify

Compares 128 bytes of data, starting at address 0x80 and ending at 0xFF to the data actually written to flash memory, performing a verification check. The same alignment requirements and warnings as with Flash Memory Write apply here.

Entry Address
ORG 0x110

Arguments

Address	Name	Function
0x7D	fmadd_bank	Flash Bank (must be 0)
0x7E	fmadd_h	Starting address upper byte
0x7F	fmadd_l	Starting address lower byte
0x80-0xFF	input data	Data to be written to flash

Return Values

ACC Value	Meaning
0x0	No data mismatch
0x1-0xFF	Data mismatch (any other value)

Clobbered Registers

• ACC
• TRL
• TRH

Flash Memory Read

Reads a single contiguous page of data (128 bytes) from Flash, starting at the specified address.

Entry Address
ORG 0x120

Arguments

Address	Name	Function
0x7D	fmadd_bank	Flash Bank (must be 0)
0x7E	fmadd_h	Starting address upper byte
0x7F	fmadd_l	Starting address lower byte

Warning

The fmadd address must be 128-byte page-aligned (bits 0-6 of the lower address reset to 0). Only a single full page can be read from at a time.

The routine requires flash bank be set to 0 or "normal operation is not assured."

Return Values
Read data (128 bytes): 0x80H to 0x0FF

Clobbered Registers

• ACC
• TRL
• TRH

Warning

This routine does not return any sort of error upon receiving invalid arguments, so there is no feedback if they are specified incorrectly. Make sure to call the routine with STAD set to 0x00, or part of the screen will be overwritten.

Clock Update

Entry Address
ORG 0x130

Arguments None

Return Values All values returned are stored in bank 0.

Address	Name	Description
0x17-0x18	year_h	Year (hex)
0x19	month_h	Month (hex)
0x1A	day_h	Day (hex)
0x1B	hour_h	Hour (hex)
0x1C	min_h	Minute (hex)
0x1D	sec_h	Second (hex)

Clobbered Registers

Mode Change

Yields control back to the BIOS, which changes modes (to clock mode) when the MODE button is pressed.

Entry Address
ORG 0x1f0

Arguments None

Return Values None

Clobbered Registers N/A

System Variables

The following is a listing of all known system variables with their description

Warning

These values are used by the BIOS to peform regular system operations and are not meant to be manipulated by the user, so writing invalid data to them can cause system instability.

Address	Name	Description
Date & Time
10	EVMU_ADDRESS_SYSTEM_YEAR_MSB_BCD	BCD-Encoded Century (first two digits)
11	EVMU_ADDRESS_SYSTEM_YEAR_LSB_BCD	BCD-Encoded Year (last two digits)
12	EVMU_ADDRESS_SYSTEM_MONTH_BCD	BCD-Encoded Month (1-12)
13	EVMU_ADDRESS_SYSTEM_DAY_BCD	BCD-Encoded Day (1-31)
14	EVMU_ADDRESS_SYSTEM_HOUR_BCD	BCD-Encoded Hour (0-23)
15	EVMU_ADDRESS_SYSTEM_MINUTE_BCD	BCD-Encoded Minute (0-59)
16	EVMU_ADDRESS_SYSTEM_SEC_BCD	BCD-Encoded Second (0-59)
17	EVMU_ADDRESS_SYSTEM_YEAR_MSB	Century (first two digits)
18	EVMU_ADDRESS_SYSTEM_YEAR_LSB	Year (last two digits)
19	EVMU_ADDRESS_SYSTEM_MONTH	Month (1-12)
1A	EVMU_ADDRESS_SYSTEM_DAY	Day (1-31)
1B	EVMU_ADDRESS_SYSTEM_HOUR	Hour (0-59)
1C	EVMU_ADDRESS_SYSTEM_MINUTE	Minute (0-59)
1D	EVMU_ADDRESS_SYSTEM_SEC	Second (0-59)
1E	EVMU_ADDRESS_SYSTEM_HALF_SEC	Half Second (0-1)
1F	EVMU_ADDRESS_SYSTEM_LEAP_YEAR	Leap Year (0 = no, 1 = yes)
General
30	EVMU_ADDRESS_SYSTEM_MODE	Index for BIOS Mode
31	EVMU_ADDRESS_DATE_SET	Date/Time Valid (0x0 = no, 0xFF = yes)
50	EVMU_ADDRESS_SYSTEM_QUART_YEAR_MSB	Quarter Year MSB
51	EVMU_ADDRESS_SYSTEM_QUART_YEAR_LSB	Quarter Year LSB
60	EVMU_ADDRESS_SYSTEM_CURSOR_POS_COL	Menu Cursor column position
61	EVMU_ADDRESS_SYSTEM_CURSOR_POS_ROW	Menu Cursor row position
6D	EVMU_ADDRESS_SYSTEM_GAME_LAST_BLK	Last block used by minigame
6E	EVMU_ADDRESS_SYSTEM_BATTERY_CHECK	Check Low battery (0x0 = yes, 0xFF = no)
Stack
80	EVMU_ADDRESS_SYSTEM_STACK_BASE	First stack location
81-FE	Middle Stack
FF	EVMU_ADDRESS_SYSTEM_STACK_END	Last stack location