SD Card issues

[Nurgle Rider]

Hi all,

   I'm new to the community just having finished building my Small Computer Central sc792 build as my first every kit.  I had expected to experience issues based on my pretty bad soldering skills but to my surprise the entire system works properly except one aspect of the SD card.

The cpu module for this machine has a dedicated serial uart for an SD card module it comes with.  The smallest SD card I own is 8GB and I had read that RomWBW can only address up to 4GB of space on drives so I created a single 4GB FAT partition.

RomWBW does detect the SD card and even recognizes it as an 8GB card:

SD: MODE=SC OPR=0x0C CNTR=0xCA TRDR=0xCB DEVICES=1
SD0: SDHC NAME=00000 BLOCKS=0x00ED6000 SIZE=7596MB

And it shows up as a drive:

Disk 4      SD0:        SD Card           7596MB,LBA

When booting into zsdos (zsystem) it configures the following drives:

        C:=SD0:0
        D:=SD0:1
        E:=SD0:2
        F:=SD0:3
        G:=SD0:4
        H:=SD0:5
        I:=SD0:6
        J:=SD0:7

I then CLRDIR drive c: which worked perfectly and it STAT shows it as an 8MB drive as expected.

However drives D through J results in: Invalid Drive Specification

And of course DIR the drive results in :

ZSDOS error on D: No Drive
Call: 17  File: ????????.???

I was under the impression that the listed drives by ZDOS were already assigned but just to be sure I ASSIGNED D:=SD0:1 and get:

   D:=SD0:1
  1859 Disk Buffer Bytes Free

I'm not sure where the 1859 number comes from but regardless this makes no difference to accessing that slice.

I have read the RomWBW chapters on drive configuration but there are several things not clear to me:

1. The documentation directs prepping drives with FDISK80.  I would assume this creates a CP/M formatted slice which doesn't make sense to me on a partition that is FAT.  In addition, why would this be needed for the further slices but no drive C: slice 0?

2. The documentation states that slices are not the same as partitions and the SCC documentation on prepping an SD card basically says create a FAT partition and you are good to go.  I'm perfectly fine creating multiple smaller partitions if that's needed but again, I'm very confused on the correlation of a FAT partition to how RomWBW/ZSDOS interacts with that.

3. Finally I'm also unclear on how user spaces work in a given slice.  Is the 8MB divided down into space for any user area that takes space or is more disk space assigned to each user area (i.e. each user area has 8MB?  That doesn't seem likely)

I apologize for having to ask these questions.  I did spend many hours trying to find an answer but it seems that most everyone uses either CF or IDE rather than SD cards.  Is that a better option?

Also I have basically no experience with CP/M.  I was loaned a dual 8" floppy drive CP/M system from my new employer in 1987 for a couple of months but obviously nothing stuck in my head :-D

I have no doubt that I'm just not doing things the right way.  Any help would be appreciated.

Regards,

Larry

[Wayne Warthen]

Hi Larry,

Nothing wrong with your SD Card interface and RomWBW handles much larger capacities than you are using -- no issue there.

I think you need to review the RomWBW User Guide.  Specifically, the Disk Preparation section.  I strongly recommend that new users start by writing a pre-built disk image to their media.

I will point out that you are creating a FAT partition whereas RomWBW needs a RomWBW partition.

I don't have much time now, but if you have more questions after reviewing the User Guide, please ask.

Thanks, Wayne

[Nurgle Rider]

Hi Wayne,

   Thank you, you cleared up my confusion with the statement "whereas RomWBW needs a RomWBW partition".  I did read the User Guide but I was biased with my FAT partition centric view.  It doesn't help that I'm sick atm and not exactly running on all cylinders.  Not that I'm all that bright to being with :-)

I imaged hd1k_combo to my SD card and all works as expected.  Now that I have a better understanding I can setup my SD cards per the guide without my confusion getting in the way.

Thank you so much for your help.

Regards,

Larry

[Mark Pruden]

Hi Larry

Welcome to the Community. It sounds like you may be on the right track. In response 

> 1859 Disk Buffer Bytes Free

This refers to number of Bytes of available Ram (in a buffer area) remaining to assign drives. Each drive letter assigned consumes some of this space based on the type of drive assigned

> I'm also unclear on how user spaces work in a given slice.  Is the 8MB divided down into space for any user area that takes space or is more disk space assigned to each user area (i.e. each user area has 8MB?  That doesn't seem likely)

User spaces are a CPM file system attribute, nothing to do with romWBW. See cpm docs. In short each file is owned by a user (0-15). Typically in the OS you  switch to a User, and a DIR only shows files assigned to that user. Think of them as 16 virtual directories per drive. 

Mark

[Wayne Warthen]

On Monday, July 14, 2025 at 6:10:11 PM UTC-7 Nurgle Rider wrote:

   Thank you, you cleared up my confusion with the statement "whereas RomWBW needs a RomWBW partition".  I did read the User Guide but I was biased with my FAT partition centric view.  It doesn't help that I'm sick atm and not exactly running on all cylinders.  Not that I'm all that bright to being with :-)

Many people fall into that trap.  It is human nature to do what has worked in the past.  CP/M seems very much like MS-DOS at first, so it is easy to think that way.

I imaged hd1k_combo to my SD card and all works as expected.  Now that I have a better understanding I can setup my SD cards per the guide without my confusion getting in the way.

Glad you worked it out.

-Wayne