MRDs provide a powerful method of modifying every detail of how the computer works. You can even replace existing commands with MRDs, or expand com- mands to make the system more powerful. Use of MRDs are explained in the User Disk Manual , and we try to explain how to write them in the Technical Reference Manual . Some common MRDs are the ones that blank the display when it is left unattended ( crtsav ), the one that displays the time ( tdos ), and the one that provides an EGA and extended graphics display ( sseg ). If the command did not refer to an MRD, the command name is compared with the 60 or so which are implemented within the 1616/OS program.
These are the commandslistedwhenyouusethe help command. Ifamatchisfound,theselected function is executed. The actual MC68000 microprocessor code to perform the selected command is contained within the 1616/OS EPROM chips. This sort of command is referred to as an ‘inbuilt command’. It is these commands that we describe in this manual. If no match is made with the inbuilt commands then the operating system searches for disk files with names based upon the command which you typed in. If, for example, you typed in ‘ diskcopy ’, the following searches occur: diskcopy.xrel ’inyour current directory(thedirectorythat appearsin your prompt). diskcopy.shell ’ in your current directory.
diskcopy.exec ’ in your current directory. If none of these are found, then a search is made for these files on other directories, as specified by your current execution path (see the xpath command for details). Most users make up a special directory, usually called bin , to contain programs they often use. Upon starting the system, they use something like the command xpath + /f0/bin to ensure that the directory /f0/bin is always searched, as well as their actual current directory. If one of these files is found, it is executed. xrel and exec files contain executable MC68000 machine code.
This sort of command is referred to as a ‘transient command’, because the microprocessor code for it is loaded into memory from a disk device only when it is needed. The shell file contains a list of commands (such as you would type in from the keyboard), which the system interprets. Many of these disk based commands are discussed in detail in the User Disk Manual . Input / output redirection The optional input / output redirections ‘ > ’ and ‘ } ’ in the command line format described at the start of this section will get input from, and send output and error messages to, the nominated character devices or files for the duration of the command.
Using the doubled redirections >> and }} means that instead of over- writing the previous output, new output is added to it. Command Handling User Tutorial Manual Page 25