- Joined: 01 Oct 2014
- Posts: 27
- Location: Sunnyvale, California
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BIOS 1.0 fully commented disassembly
 Posted: Fri Mar 20, 2015 7:37 am
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Salient differences from BIOS 1.3:
* No animated SEGA / Master System bumper screen, it just appears
* Data for the SEGA and Master System logos are stored uncompressed (the Phantasy Star Tile Decoder code is not present)
* It doesn't store the commonly used VDP routines (WriteVRAMByte, etc.) at the beginning of the ROM to take advantage of the rst opcodes
* The detection code is copied and called earlier, and instead of the fixed wait of ~1.67s in 1.3, there is a variable wait based on the size of the ROM detected: no ROM: ~1.67s, 8KB => 1.55 s, 16KB => 1.43 s, 32KB => 1.19 s, 48KB => 0.95 s, 64+KB => no wait
* If no ROM is detected, it displays a simple error screen that just contain the strings "MASTER SYSTEM (c) SEGA 1986", "S/R V1.0", and a blinking "?"
* If the checksum test fails, there is an extra "CHECK SUM ERROR" string before the "?"
* The ROM detection port is the same as 1.3, except it doesn't read port $dc, and if we fail the region check, it sets ROM_HardwareSlot to 0, instead of setting the checksum in the copied ROM header to the computed checksum + 1 (in other words, 1.0 doesn't distinguish between "no ROM" and "invalid region ROM")
* No reset button handler
* The frame interrupt handler is never called, as the interrupt is never enabled
* The entire sound engine is there, and is almost exactly the same as in 1.3. It is never called though...
* The snail maze game songs are all present, and are almost exactly the same as in 1.3, even though the rest of the game is not there
* There is an early version of the song that plays during the animation of the bumper screen of 1.3, but it doesn't sound right/finished
So overall, this looks like a WIP version of 1.3, rather than a polished 1.0 :)
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