Error codes

Basic DERR command returns the last error code after the failure of a read or write file command1). UniDOS uses the very same error codes than AMSDOS, and like AMSDOS does, 7th bit let you know if the error was already reported or not, and 6th bit is set in case of low level error2).

In addition, UniDOS introduces some new error codes which are explained in the list bellow (added in bold to the AMSDOS errors):

Error AMSDOS/UniDOS DERR value Error description
0 0 ou 22 [ESC] was pressed.
14 142 (128+14) Invalid channel state.
15 143 (128+15) Hardware end of file.
16 144 (128+16) Bad command.
17 145 (128+17) File already exists.
18 146 (128+18) File does not exist.
19 147 (128+19) Directory full.
20 148 (128+20) Media full.
21 149 (128+21) Media changed while files were opened.
22 150 (128+22) File is read-only.
23 151 (128+23) Path is a directory.
24 152 (128+24) File is not a symbolic link.
25 153 (128+25) Access type not supported by this physical drive.
26 154 (128+26) Software end of file.
27 155 (128+27) Path is a file whilst a directory was expected.
28 156 (128+28) Attempt to rename a file accross two different physical drives.
29 157 (128+29) Move file not supported by this physical drive.
30 158 (128+30) Real time clock not found.
31 159 (128+31) Real time clock is read only.

LOAD/SAVE over directories

When using the BASIC command LOAD or SAVE to change the current directory or to create a new one, it is returning a silent error (without any displayed message) to prevent the Basic from actually loading or saving anything (because obviously, no file at all is involved).

If it is harmless with direct input, this error will interrupt the execution of any Basic program issuing a LOAD or a SAVE on a directory, just like if a file was not found or of a bad type, with the notable difference that no message is displayed. If this behavior is not what you expect, you can simply use the |LOAD and |SAVE RSX instead of the Basic commands; those will not generate an error and the program will not be interrupted.

20 PRINT"The current path is:":|PATH

If you want to use the LOAD and SAVE anyway, in case of an interactive program, you can actually catch the generated error with the ON ERROR GOTO command, and then determine if the user selected a directory, a file3) or if an error occured. Of course, you will have to take care of this in any Basic program dealing with directories through LOAD and SAVE commands. But if you just want to ignore the silent error related to directory change, here is an example:

100 '
120 PRINT"The current path is:":|PATH


UniDOS is, thanks to its paths and drives smart handling, strongly compatible with softwares designed for AMSDOS. And because of that, UniDOS is already able to run most major softwares from any modern drive whereas they were only planned to be launched from floppy discs. Almost all games in file versions will run under UniDOS, even the ones which are very restrictives regarding memory.

Please note that UniDOS is not CP/M compatible; all programs that have to be started using |CPM RSX will still require to be stored on floppy disc.

Non exhaustive list of compatible softwares

Even if they are compatible with UniDOS, most of the programs in the list bellow will only work if you are using the optimal installation4). Please also note that all games and demos available in files5) should work in this configuration.

  • AMX Pagemaker,
  • Contiki,
  • Digitracker6),
  • Maxam,
  • Orgams,
  • Protext,
  • Protracker7),
  • Soundtracker DMA,
  • Starkos8),
  • The Advanced OCP Art Studio,
  • The Soundtrakker9),
  • Utopia (most of the tools),

Non exhaustive list of incompatibles softwares

In addition to the softwares exclusively designed to work from floppies and using trackloads, some utilities apparently system friendly do not work (yet)10).

  • DES,
  • Oxford PAO.


Here are some tips and advices to use UniDOS at its best with applications initialy designed for AMSDOS and which obviously do not know how to deal with directories and drives (apart of legacy A: and B:).

Most softwares such as “The Advanced OCP Art Studio”, “Starkos” or “The Soundtrakker” are filtering file names depending on their extension so that they will only show you files they can handle. To be able to use directories anyway, you will just have to rename them with the expected extension (.SCR, .WIN, .PAT, .FNT, .SKS…). An error will be generally displayed when trying to load them, but it will be harmless and the directory will be actually changed!

That said, it will not work with some softwares (such as “The Soundtrakker”) because they do not properly handle file attributes or are also checking their size (directories are listed as protected file or size zero). Also, you won't be able to go to the parent directory, which can be annoying. In both cases, you will just have to use the symbolic links as explained bellow.

Symbolic links are very powerful objects which can be used in a number of cases. Actually, they will allow you to create anywhere entry points to any file, directory or drive. And when dealing with old softwares designed for AMSDOS, they can ease directory navigation.

For instance:

  • Creating, in the directory of a software, a symbolic link which points to the location where are stored your project files will let you access them from its interface.
  • Creating, in a sub-directory, a symbolic link to “/” will let you go back to the parent.
  • Creating a symbolic link to another drive will let you change the assignation of the current logical drive (A: or B:) without having to leave the software.

UniTools and developper tool ROMs

Thanks to its RSX |DEVTOOLS.ON and |DEVTOOLS.OFF, UniTools can activate (default state) and deactivate developper tool ROMs at CPC boot. To do so, ROMs considering themselves as developper tools must check a memory tag during their initialization phase. It can be done as follow:

; Code to insert at the beginning of the init routine of developper tool ROMs

MagicNumber         Equ &feca
DisableROMTagAdr    Equ &baf7

        push hl
        ld bc,-MagicNumber
        ld hl,(DisableROMTagAdr+0)
        add hl,bc
        ld a,h
        or l
        pop hl
        jr nz,ROM_ActualInit
        ld a,(DisableROMTagAdr+2)
        and &40
        ret nz ; NC
        ... ; Actual ROM initialization code

Please note that Arnor's ROM proposed in the download section (Utopia, Protext and Maxam) have been patched to be considered as developper tools.


Here are some numbers to give you an idea to what you can expect performance wise.

Numbers given bellow are average on reading and writing 16 KiB files; time required for file system management is then included, which gives realistic speed for standard usage rather than burst values you will never reach.

(ROM overlay)
Floppy disc drives 10 KiB/sec 10 KiB/sec 9 KiB/sec
Albireo - MicroSD card11)  70 KiB/sec 25 KiB/sec 16 KiB/sec
Albireo - USB storage12)  86 KiB/sec 23 KiB/sec 13 KiB/sec
M4 Board - MicroSD card 107 KiB/sec 50 KiB/sec 43 KiB/sec
FatFs - X-Mass 58 KiB/sec 18 KiB/sec 12 KiB/sec
Theorical maximum 120 KiB/sec 120 KiB/sec 80 KiB/sec
it is related to “CAS” low level routines return codes
for more information, please read the Amstrad CPC user manual, chapter 6, section “AMSDOS floppy disc error”
when an explicit path was provided
ideally with UniDOS ROM in slot 7 and AMSDOS/ParaDOS ROM in slot 15, or by using the Firmware patched “u5”
officially or not
6) , 7) , 8) , 9)
optimal configuration required
notably because of non documented direct accesses into the ROM
11) , 12)
depending on USB storage type and microSD card model, but also on file system fragmentation, these numbers may vary for about 10%