PPCenter :: devblog

PPcenter. Arts and craft for my Sega Saturn. Since 1847 :D

Bit shift in Saturn minimalistic C program

Written by cafealpha 4 comments
While making a Saturn program running directly from ROM, constraint is usually to have program as small as possible. Why ? Because I would like to make Pseudo Saturn Kai continue supporting Action Replay, which have limited ROM size

So, in that situation, I use homemade, minimalistic stdlib, containing usually only memcpy and memset functions, and everything goes well.
But for some reason, current program was begging for __ashlsi3 and __ashrsi3 functions.
... What's these things ? __ashlsi3 is an assembly function doing left shifts of data, and as you probably guessed, __ashrsi3 does the same, but with right shifts

No problem, let's ask my friend google, and copy & paste theses functions without minding about their contents Theses functions are standard functions provided by C library, so sources can be found here and there.


An example of what __ashrsi3 function looks like

A day after that, here comes the time to do basic testing of this ROM code
Let's first start with simple things such as screen display ...


Text display ... not as expected ?!

Hrm, it's probably a problem in my printchar routine ?

printchar source code : this is the core thing
behind text display in my programs


But that's a very long time I didn't touched this routine, which works well on Pseudo Saturn Kai, and works even better in vdp1ex from Charles MacDonald sample programs, because all my printchar routines come from there
But just in case of, let's try to tune one " <<3 " into " *8 " in the code above, and see if that changes something. Theses two operations are equivalent, so this should give the same result, but ... Saturn screen becomes all black

Okay, there's definitely something weird regarding that printchar, but I just re-used it as-is from other project, so what's going wrong with my it ?!
[...]
And then (finally) a light-bulb above my head appears Maybe there's something wrong regarding the __ashlsi3 and __ashrsi3 routines ?
And, yes there was something wrong regarding theses routines ... I wrote a paragraph about them on the first half of this article, so it would be surprising the problem came from somewhere else

In more accurate words, routines are OK, which is normal since they are written by people smarter than me But the calling convention of __ashrsi3 was not as my gcc was expecting ?
Well, that's just a guess from the display results, because it seems to display the same pixel 8 times on each row of a given character, which may happen when ignoring shift parameter in __ashrsi3 function.

Anyway, rather than finding in details what's wrong, let's fix the problem First, with assembly by hand, in desperately trying to change input parameter handling ... I don't remember exactly what I wrote, but that was something like pushing a register to stack, moving parameter to it, finally restoring register, putting nop instructions everywhere, etc ... nothing difficult, but over my extent in assembly programming

The result is ... a mixed success :

Trying to modify __ashrsi3 routine by hand ... well, it seems that
some shift values are not correctly handled


Okay, so let's re-ask google about that __ashrsi3 routine ... and a different implementation arrives in search results. Let's try it


__ashrsi3

And the execution result :

Execution result with second version of __ashrsi3 function

Yeah, it works this time !!! And don't ask me why, because I didn't took time in understanding what's different in that second version of __ashrsi3 function
And, rather than understanding why it works ... let's close that assembly pandora box before some other mystic bugs pop from it

I will probably see about this in the future, but that was absolutely out of the scope of today's programming, and in software world, too much digression leads to freeze of projects, which I would like to avoid in order to go forward to next pending project 

As a example, I started to adapt yeti3d engine to Saturn in 2010 (7 years ago !), and this ended in ... developing a SD card based memory cartridge for Saturn !
The point above is is not a joke : menus used in my yeti3d adaptation are the origin of menus used in Pseudo Saturn Kai, which shows the continuity (co-consanguinity ? ) in my projects. And after getting yeti3d working a bit, I really would like to load levels from something else than CD-ROM, which was from PC (via adequate link cable) on a first time, and which will (should ?) then evolve to SD card.
That would be cool to make a Saturn game not requiring to burn CD-ROM somedays, but before that I need to finish neighboring side quests


Evolution of my Saturn projects

To conclude this article, let's say it was terribly fun to see some bits of the internals behind C language
By the past, I remember I did something similar with my yeti3d adaptation, but remained at "C language level" : optimization done at that time was (IIRC, ) to avoid 4 bits shifts, because theses are not available in a single SH-2 CPU instruction. Avoiding 4 bits shifts was simply done by merging two fixed point operation in a single one (or something like that : I did this 7 years ago !), and this actually gave some improvements in 3D scene rendering Theses were the good times ... I sometimes think "I'm Getting Too Old For This Shit", but that's only to motivate to finish my old projects

But before that, Pseudo Saturn Kai, and then Kicad and Quartus are waiting for me


BTW, in the case you wondered about what was today's programming session, you probably guessed it was about exception handler addition to Pseudo Saturn Kai. This not a new feature, since it was available in Pseudo Saturn 0.83x ... in fact, I grabbed some sources from Pseudo Saturn 0.83x in order to implement this exception handler (thank you CyberWarriorX !) This exception handler will be available as a small add-don to cheat codes feature in Pseudo Saturn Kai next major release.

Read more Bit shift in Saturn minimalistic C program

Cartridge labels available !

Written by cafealpha no comments
Finally available :]

Last June, I initially wanted to release Saturn Gamer's Cartridge with custom label, but some unexpected problems regarding label quality (not resistant to water) delayed its release, consequently, Gamer's Cartridges manufactured until now had only shell, but no label.

"Stronger" label retained for use by Gamer's Cartridge is designed for car tuning purpose, so waterproof, and even alcohol proof ! I suppose this will satisfy the requirements of typical Gamer's Cartridge user
(※) Just a (common sense) note : while labels are waterproof, cartridges themselves aren't  So no, you still can't play Saturn while taking a bath

data/images/20160914_labels.tb.jpg
Cartridges with their labels.
Bottom-right one (without LEDs) is USB dev cart.


Custom labels were the last main milestone regarding Gamer's Cartridge design : I won't design custom boxes for my cartridges.
Why ? Because I want Gamer's Cartridge users to ... use their cartridges, not collect (shelf) them

You are Saturn Gamer ? Want to share your Saturn saves with your emulator or friends ? Want to enjoy Dezaemon 2 game but don't want to purchase a modchip + memory cartridge ? Then Saturn Gamer's Cartridge is for you


Special thanks to my friend Darius who selected paper, printed labels sheet, and cut each labels
Darius provided pictures he took when cutting labels, so please enjoy the labels making-of


data/images/20160818_labels_scan.jpg
Label sheet scanned just after print-out.
Red/blue lines are used as help when cutting labels.



data/images/20160819_labels_tools.jpg
Tools used : ruler and cutter.
And also Darius' God Hand and eyes (not pictured)


data/images/20160819_labels_row_cut.jpg
In order to optimize time , labels are first cut row per row.

data/images/20160819_labels_individual_cut.jpg
After row cutting, labels are cut individually

Read more Cartridge labels available !

Could finally write a file to my SD card :]

Written by cafealpha 3 comments
I could finally write a file to my SD card ... from my Saturn
Well, I just tested by dumping my Saturn's BIOS, and didn't tried further, because I don't want to find and fix other bugs today

data/images/20150716_bios_dump_ok.tb.png
Top : my Saturn BIOS, dumped from my Freewing device 5 years ago
Bottom : BIOS file in my SD card dumped today
Hex Editor says that both files are identical

This fixes a nearly 3 years old bug
At that time, I connected a SD card to my second pad port on my Saturn and could read files successfully. However, file write sometimes failed ... I thought about many possible causes of failures (not enough 3.3V power supply ? Bad luck ? etc etc), but it was a software bug : writing sectors to SD card actually takes longer than reading sectors, and I didn't waited enough for my SD card to finish write.
Changing poll timeout count from 500 clocks to 50000 clocks seems to fix the problem. (Q&D fix, but if it works fine, I won't change it )

So now, as SD card read/write access works on both yabause and real hardware, it is time to to use it in order to develop new features for my Saturn

data/images/20150716_memcart_rev2f.tb.jpg
My SD card cartridge for Saturn (Q&D prototype revision )

Edit : wow, current cartridge firmware build ID (incremented on each make/build) is 4200

Read more Could finally write a file to my SD card :]

Rss feed of the tag