68881 FPU Testing Thread

Started by RobIvy64, February 07, 2012, 01:12:14 AM

Previous topic - Next topic


Just curious about this one.

I have an FPU board on the way, and would like to know what games or software takes advantage of the 68881.

I doubt many games do, but I am sure there are some.
"Console Mods" lurker


Cho ren sha uses Float2.x.  And I think that's it.


I knew about Cho and Otoko Dama

It's worth having for cho ren sha alone.
"Console Mods" lurker


"Console Mods" lurker


So for shits and giggles, I deleted FLoat2 off of Cho Ren Sha, added float3, then edited the config.sys to use float3.
Normally I run the game at 16mhz, and there is a tiny bit of slowdown here and there.  Using Float3, the slowdown
was totally removed, but the game flickered like crazy.  Enemies, bullets, flickered at times like a cheap transparency effect. 
Weird as hell, and not recommended lol. 


I'll be testing this out myself with a number of games, including retail releases.

My FPU board should be in soon  8)
"Console Mods" lurker


OK, I finally had time to play around with the FLOAT3.x FPU driver.

I did the same thing that deadite did in regards to Chorensha 68K.

First, I removed FLOAT2.x in config.sys, and replaced that line with FLOAT3.x

The results? No change. Slowdown appeared to be the same, and no graphical errors like deadite reported seeing.

I added the line for FLOAT2.x back into the config.sys, and again the same results.

To ensure that it is using the hardware FPU, I am going to remove the FPU board, and boot Chorensha using only the FLOAT3.x driver.
"Console Mods" lurker


Chorensha will not start when FLOAT3.x is used in place of FLOAT2.x and no 68881 is present.

With the 68881 installed, it starts as normal.

Here is the error if anyone would like to translate it.

"Console Mods" lurker


This is the typical "BusError" message from human68k. This is an exception raised by the cpu when a program try to access data somewhere in address space, but there is nothing at this address. In your case, the FPU.

From what I know :
- FLOAT2.X is a softfloat driver, a way to emulate IEEE FPU instructions using m68k integer arithmetic instructions.
- FLOAT3.X is the driver for the real FPU (68881 or 68882) as an expansion card.
- FLOAT4.X is the driver for M68020+ which embed an FPU.


Here is the description of Float3.x from the FPU manual.

"Console Mods" lurker



この数値演算デバイスドライバは、FPC を利用して数値演算を処理するデバイスドライパです。

Math device driver is a device driver to handle the math performance using the FPC.


If you are using this board (not incorporated in the CONFIG.SYS file in the disk supplied.) Usually use this device driver.


Internal representation of floating-point number is in compliance with IEEE Fuomatsu me.


What message is displayed when the system boots beneath Yolo and incorporate this device driver.

浮動小数点演算 Co-PROCESSOR DRIVER for X68000 version 1.00 (IEEE フォーマット)

Co-PROCESSOR DRIVER for X68000 version 1.00 floating-point arithmetic (IEEE format)


Placement of the system on the memory of when incorporating the device driver will be as under.

X68000 personal computer is called, "X68K" or "no good good" is called, is the PC that are loved by many people today.


Quote from: SuperDeadite on February 07, 2012, 01:16:59 AM
Cho ren sha uses Float2.x.  And I think that's it.
I know it's an old thread but probably this might be interesting: Cho Ren Sha uses floating point functions only for data table initializations and the CPU benchmark at start. Also Cho Ren Sha will use a different sprite display mode when the "CPU_POWER" is 130% or higher.



I'm posting to this thread to keep knowledge under the same subject.

I have a KGB-PRKII expansion with an FPU socket.

Ok, I purchased one FPU and placed into its socket. Pictures here: http://goo.gl/tr0kjX

Also I noticed that there's a jumper, which I changed the position according to other KGB+FPU pictures I saw.

Is that jumper position right?

Besides that, reading these FPU threads I know that I need to change CONFIG.SYS, replacing FLOAT2.X by FLOAT3.X

I think that's it, any other hint will be appreciated!



I'm not sure about the KGB PRK memory board with the optional FPU slot..

But here's your FPU drivers in a nutshell:

Float2 emulates the '881/2 FPU's in software, using the 68k CPU's instructions alone. witch makes it slower than the hardware FPU drivers.

Float3 uses the real '881/2 FPU's.

Float4 is a third party FPU driver, for the 68040's built in FPU. I'm not sure if this works with a separate '881/2 FPU though. 


To this day, Xak II and Crossfire EX are the only games I know of that officially support the 68881.


If you change your config.sys to load the float3.x and the jumper isn't correct, it will throw an error. If I get a chance this weekend, I'll pull my KGB-PRKII and see how it's set as I know it's working.


I did the float3.x change in config.sys and the X68 started with no issues!  :D