Serial device mapping in CP/M

terminal prompt This article is about how CP/M handled serial input/output. This is a complicated subject, but perhaps one that is not only of interest to retrocomputing enthusiasts. Comparing how modern operating systems handle with serial devices with how CP/M did it can provide a useful insight into operating system design in general.

The problem

Many serial devices existed in the CP/M days -- terminals, network connections, modems, tape storage devices, printers... most of these still exist. A well-behaved CP/M program would do I/O using functions in the CP/M BDOS (operating system). BDOS would relay these function calls to the BIOS. However many serial devices were connected to the computer, the CP/M BIOS provided an API (application program interface) for exactly four devices, and no more. As an additional complication, only one of these devices was bidirectional. So how could the CP/M system interact with more than four devices?

The CP/M 'logical' devices

The CP/M BIOS recognizes four logical devices. The term 'logical' denotes that they aren't necessarily tightly bound to specific hardware. Here they are, in their entirety:

the CONSOLE device, usually referred to as 'CON:' in CP/M, which could be written or read;

the LIST device, usually referred to as 'LST:' in CP/M, which could only be written;

the PUNCH device, usually referred to as 'PUN:' in CP/M, which could only be written;

the READER device, usually referred to as 'RDR:' in CP/M, which could only be read.

The original function of the CONSOLE device should be fairly clear -- it was nearly always associated with the user terminal. LIST was traditionally a printer device, so it didn't need to be read. PUNCH and READER originally referred to a paper tape writer and reader. Typically these would have been different machines, so there are different devices with different APIs. This is different from the CONSOLE device -- it was assumed that reads and writes on the console device would go to the same place. So some devices are bidirectional and some are unidirectional.

Of course, it's a long time since anybody seriously used paper tape for storage. The PUNCH was already considered an 'auxiliary' device by the time CP/M was established. It might have been assigned to a tape storage device, for example, or to a modem.

The BIOS APIs that did input and output to these various devices were mirrored more-or-less exactly in the CP/M BDOS. So there were BDOS functions to read and write the console, read and write the 'punch', and write the list device. It's important to understand that there was no portable way for an application to do serial I/O except to use the four standard devices via their specific BDOS APIs. Hardware vendors might have provided extensions with more flexibility, but portable software could not rely on this.

The 'physical' devices

Although the BIOS only provided APIs to write to the four standard logical devices (CONSOLE, LIST, READER, and PUNCH), it recognized additional devices, even though an application could not address them directly. These were typically referred to as 'physical' or 'hardware' devices, because the four logical devices could be mapped onto them in particular ways. Most vendors recognized twelve of these physical devices. For the record, they are known in CP/M as BAT, CRT, LPT, PTP, PTR, TTY, UC1, UL1, UP1, UP2, UR1, and UR2. It's hardly worth explaining what all of these names meant, because they were archaic, and rarely corresponded to any recognized device. So, for example, 'UP' is 'user punch'. Having even one paper punch was unusual in the CP/M days; it seems highly unlikely that any system would have had an additional punch device. 'CRT' (cathode ray tube) makes at least some kind of sense -- this name usually denoted the user console device. I will explain some of the other names when they come up.

In practice, with the possible exception of CRT, the meanings of all the device names was (and is) system-dependent. Moreover, it's unlikely that any CP/M system of the 1980s (or even now) would have twelve different serial devices attached; multiple names mapped onto the same physical port. Sometimes the different names referred to the same hardware, used in slightly different ways. Some of the physical devices (like UP) were unidirectional; some were bidirectional.

Mapping logical to physical devices

If the BIOS and BDOS only allowed access to four serial devices, how can an application use all the other, 'physical' devices? This is achieved by mapping the physical devices onto the four logical devices. It would be nice if any of the twelve physical devices could be mapped to any of the four logical devices, but CP/M was never that flexible. Instead, a single, 8-bit byte controls the assignments of all four logical devices. This piece of data is conventionally called 'IOBYTE', and is stored at address 0003 in the CPU's address space. Four pairs of bits control the mappings of the four logical devices, according to the following table.

     Device   CONSOLE   READER     PUNCH      LIST     
     Bits     0,1       3,4         4,5        6,7   

       00:    TTY        TTY        TTY        TTY 
       01:    CRT        PTR        PTP        CRT  
       10:    BAT        UR1        UP1        LPT 
       11:    UC1        UR2        UP2        UL1       

Note that the default value, zero, leaves all the virtual devices mapped to TTY, which is typically the console. The PUNCH device can be mapped to three other physical devices, all of which are historically 'punch-like': PTP ('paper tape punch') and UPx ('user punch'). The LIST device can be mapped to the console, but also to the 'printer-like' devices LPT ('line printer') and UL1 ('user lineprinter').

It's vital to understand that the CP/M did not impose any binding interpretation on the four standard logical devices or the physical devices. The UPx and URx devices, if they existed at all, would typically correspond to RS232 ports on the back of the machine. It was up to the user what got plugged into those ports. If the machine had a Centronics-style printer port, it was probably assigned to the LPT device. In the end, though, the machine vendor decided what the physical devices did. The vendor may have provided tools to configure these devices, or to assign particular names to particular ports, but there was no commonality between vendors., but there was no commonality between vendors.

Mapping serial devices in practice

Suppose, for example, that a serial modem is connected to the UC1 ('user console') port. How can I use the modem to communicate with some remote computer to which it is connected over a telephone network?

One simple method would be to change the lowest two bits of IOBYTE to the value '11'. Anything that reads or writes the standard CONSOLE device will now be using the UC1 port, rather than the connected terminal. The ROMWBW version of CP/M has a utility talk that performs exactly this assignment, and then just reads from the 'console' (however that is mapped) and echoes the results back to the console. So, for as long as this program is running, data to and from the local console will be relayed to the remote system via the modem.

How do we do this in modern computers?

The modern computing industry has been so thoroughly infiltrated by the Unix way of doing things, that sometimes it's hard to imagine that there is more than one way to solve a particular problem. One of the key design philosophies of Unix is 'everything is a file'. The Unix founders recognized that things like 'read' and 'write' are generic operations, which can be applied to a huge range of hardware. 'Writing' to a serial modem is implemented very differently from writing to a file on disk but, in the end, both operations amount to sending data to some piece of hardware. A programmer should not have to worry about the particularities.

So a piece of program code for Unix (and its derivatives like Linux) would interact with all devices in essentially the same way. It would open a suitable driver, and then call the functions read(), write(), etc., on an integer file handle. In the simplest cases, it's irrelevant what the hardware actually is.

The developers of CP/M did not see things this way. It's reasonable to ask why we should use the same APIs to operate on different kinds of hardware that have little in common. That we don't ask is illustrative of how influential Unix has been. But the Unix approach leads to problems that simply don't exist in CP/M. For example, in a Linux console program, how can I check whether the user has pressed a key without actually waiting for keyboard data? This is a surprisingly involved process, and the complexity stems from the fact that a keyboard isn't a file. If 'everything is a file', then core functionality is based on the way files behave. Linux I/O works the same on all devices for the functionality they all have in common. Access to functionality that is specific to a particular piece of hardware is often difficult.

In a CP/M program, if I want to know whether the user has pressed a key, I just call BDOS function 6. It's really that simple. It's simple because CP/M doesn't strive for commonality between different devices. The Unix approach is very powerful, but it has its problems -- problems that did not exist in CP/M, or even in MSDOS.

Unix does not really recognize that some serial devices are unidirectional. I can write code that reads from a serial printer or a video display, and the results will be unpredictable. I can't make such a mistake in CP/M -- the API calls simply don't exist in the operating system. There are BDOS functions to read and write the console, but there is no function to read the printer, because that isn't something it made sense to do on a printer, even though it was meaningful for some devices.

It's also worth bearing in mind that the CP/M method of determining which logical devices map onto which physical devices requires exactly one byte of configuration. We no longer have to worry much about the odd few bytes, or even the odd few megabytes, but RAM was a premium product in the CP/M days.


CP/M's way of handling serial devices is no longer well-documented, and it's quite complicated. It's worth knowing about, however, even if you're not a CP/M user, because it gives useful insight into how people thought during a crucial time in the history of computing.