MINI-UNIX is a somewhat limited variant of V6 Unix, for use on models of the PDP-11 such as the PDP-11/05 which did not have memory management (which was required for early Unix versions from 4 onward). Although it is not full Unix, it retains a large amount of the functionality of the original.
Most V6 applications run without source modification on MINI-UNIX, although it is not binary-compatible with V6. This is a result of the lack of memory management in the machines which run MINI-UNIX; the kernel and user commands (as processes) share a single PDP-11 address space, with the kernel in low memory, and the processes (one at a time) in high memory.
There was a second, earlier, stripped-down variant of V6 for PDP-11's without memory management, LSX; it differed from MINI-UNIX in being targeted at machines with more limited amounts of main memory (a real concern with early machines, which often came with limited amounts). MINI-UNIX was targeted at machines with the full 56KB possible on such PDP-11's.
MINI-UNIX did not originally support the LSI-11, but there are no significant architectural differences between the LSI-11 and other 'small' PDP-11's, so MINI-UNIX was historically brought up on the LSI-11. (The main significant difference is that in the LSI-11, access to the PS requires use of special instructions; see here for other issues.)
That version has since apparently been lost; however, because of the wide availability of the LSI-11 in the vintage computer community, MINI-UNIX has recently been ported to the LSI-11 again, to provide a Unix experience on those machines.
Thus, 24KB (notionally) is normally allocated to hold the kernel (with a modest number of processes and disk buffers), leaving 32KB available for user commands/applications. (The "notionally" is because that 24KB also includes the 'user' structure: a per-process swappable data area, 1KB in size; thus 23KB is actually avaqilable for the kernel.)
This division can be altered, albeit painfully (all the command binaries have to be re-linked to the new boundary address), to allow the size of the kernel to vary.
The interface to the I/O system is mostly the same, so that existing V6 device drivers can basically be 'dropped in', almost un-modified. In particular, the interface to 'character (un-structured) devices is identical, as is that to the 'tty' driver; the interface to the block device buffer system (for mass storage devices) is also identical. The two minor aspects that are changed are:
- since the system is limited to less than 64KB of memory, the 'extended memory' field in block device buffer header is not present;
- 'raw devices' (which can do DMA transfers directly from the device to a buffer in the user process' memory) are not supported.
MINI-UNIX uses the standard C compiler, which generate EIS intructions. On PDP-11 CPU models which do not support the EIS, an simulation package is available which is called into action when a trap is produced by an unknown instruction.
The scheduler is a simple round-robin time-slicing one.
MINI-UNIX supports almost all the standard V6 system calls, with the excepion of:
Pipes not supported not only because of space in the OS, but because a process swap-out and -in will be required every time a different stage needs to run, leading to poor performance.
Although pipes are not supported, the shell supports the pipe command syntax ('|'), by use of temporary files (modulo the amount of free space on the disk, of course).
There is no support for command images with 'pure texts', or for so-called 'split I+D' mode.
The removal of ptrace() means there is no interactive debugging; this is not a major hardship, as the lack of memory management means that a buggy user program can crash the system anyway.
The maximum file size supported is 1MB; 'large' files are supported, but not 'huge' files (those with two levels of indirect blocks).
'Read-ahead' has also been removed from the file system implementation; this saves on address space in the kernel at the cost of a slight degradation of performance.
Only a few commands have significant changes:
- Shell - no pipes
- Linker 'ld' - minor modification to change target address
- Debugger 'db' - minor modification to change program address
A few other commands have minor changes, but they are not of any great consequence. (See Section 4 of the "The MINI-UNIX System" memorandum for details.) Some have been slightly modified to fit into the 32KB available for user processes.