Sven Petersen discovered an old EPROM marked VC-1541-DOS/80 in the UD11 option ROM socket of a Commodore CBM 8032. It turned out to be for a user port adapter that allows PET/CBM computers to use CBM IEC serial peripherals like the 1541 disk drive. Sven did not have the hardware but he reverse engineered the requirements from the EPROM and designed his own PCB. With the EPROM and Sven's PCB, you can use IEC peripherals with the PET.

This is a disassembly of the VC-1541-DOS/80 EPROM.

• BASIC 4.0. The code makes many calls that are specific to the BASIC 4.0 ROMs.

• 80-column Screen RAM. For the wedge commands !print#, !get#, !input#, and !cmd#, the code uses $87D0, which is a non-visible location in the 80-column RAM. These commands will not work on a 40-column machine, although all others will. To adapt the code for 40-column PETs, $87D0 could be replaced with $83E8.

• An 80-column Editor ROM. For the !cmd# command only, the code changes the VOUT vector to redirect output through its own routine. The VOUT vector is a feature of the 80-column Editor ROMs only. It is present in the 80-column North American and German (DIN) Editor versions but is not present in any 40-column version.

• An unknown 80-column Editor ROM. Again for the !cmd# command only, the code expects $E787 to be the default address that the Editor ROM stores in VOUT. However, in the standard Editor ROMs for North America and Germany (DIN), there is nothing at $E787, meaning VC-1541-DOS was written for use with an unknown Editor ROM. To adapt the code to the standard 80-column editor ROMs, replace $E787 with $E20C.

VC-1541-DOS provides a BASIC wedge. Install it with sys 40960 and it will show:

vc-1541-dos/80

All commands in the VC-1541-DOS wedge are prefixed with a !. The standard CBM BASIC commands can still access IEEE-488 devices but only the ! commands can access IEC devices. Although most ! commands share the same names as their CBM BASIC counterparts, the usage of these commands is not exactly the same.

Command shortcuts, whitespace, and quoting works like CBM BASIC, e.g. !load "foo" can be shortened to !lO"foo. Variables and expressions can be used, e.g. f$="name":!load $f or !print#2,chr$(x+1);. Most commands work from direct mode or from within a program, but some do not work in direct mode, like their CBM BASIC counterparts. The variable st is updated after commands but dsand ds$ are not supported.

VC-1541-DOS uses a default device number stored in location 1022 ($03fe). It is set to 8 when the wedge starts. If a wedge command is entered without a device number, such as !load"filename", then the default will be used. You can poke a new default. After poke1022,9, the command !load"filename" will load from device 9.

A wedge command may be prefixed with a device number to override the default. For example, !9load"filename" will load from device 9. The prefixed device number will only be used for the command that immediately follows it.

If bit 7 of the device number is set, an automatic CR (0x0d) to CRLF (0x0d, 0x0a) translation mode is activated. The !print# and !cmd# commands will automatically send an LF after every CR in this mode.

VC-1541-DOS can also be used without installing the wedge by entering sys 40963 followed by a command. This statement is equivalent to !load"filename":

sys40963load"filename"

All of the VC-1541-DOS commands may be used this way, regardless of whether the wedge is installed or not. Due to the way sys parsing works, a device number prefix (such as !9load"filename" in the wedge) cannot be used.

The commands !print#, !get#, !cmd#, and !close# can be used with sys40963 but they cannot be the very first VC-1541-DOS command executed because they do not initialize the VIA. This is not a problem when working with disk files, since !open is always called first. However, to read the command channel of a drive into variables without first sending a command, or to send data to a printer, !open is not called first. As a workaround, you can use sys40960:!q first, which installs the wedge (initializing the VIA in the process) and then immediately uninstalls it.

Only the first two entry points are intended to be used from BASIC. However, there are 15 additional entry points available for use by assembly language programs. These provide a "unified" or "universal" API to control both IEC and IEEE-488 devices. Please see the source code.

The original author is unknown. The EPROM contained the strings vc-1541-dos/80 and (C) g mutz (84).

Sven Petersen discovered the EPROM, disassembled large parts of it, determined the hardware requirements, and designed a circuit and PCB for it. He was the first to get it working and published extensive documentation.

Martin Hoffman-Vetter is credited by Sven for noticing that the IEC routines in the ROM are nearly identical to those in the C64 KERNAL, which greatly simplified disassembly.

To make this disassembly, I ran m740dasm on the original binary and then added symbols and comments.

No rights are claimed on the original VC-1541-DOS code or C64 KERNAL code.

All other work in this repository is made available under the 3-Clause BSD License.

Mike Naberezny