#### 2.6.2 Environment Variables for the Runtime System

These environment variables are referred to by the runtime system.

`HOME`
`HOMEDRIVE`
`HOMEPATH`

  Directory in which to look for init files. E.g. `c:\users\joe` or `/home/joe`. Under Windows, the environment variables `HOMEDRIVE` and `HOMEPATH`, set by the operating system, are used instead. Under unix, `HOME` is set by the login shell.

`TMPDIR`
`TEMP`
`TMP`

  Directory for various temporary files. The variables are tried in the given order. If none of them is suitable, built-in defaults are used: under unix, `/var/tmp`, `/usr/tmp`, `/tmp`; under Windows, `\temp`, `\tmp`, and `\` (all on the system drive).

`MITSCHEME_INF_DIRECTORY`

  Directory containing the debugging information files for the Scheme system. Should contain subdirectories corresponding to the subdirectories in the source tree. For example, if its value is `f:\random`, runtime system debugging files will be expected in `f:\random\runtime`, while Edwin debugging files will be expected in `f:\random\edwin`. By default, the information is searched for on the library path.

`MITSCHEME_LOAD_OPTIONS`

  Specifies the location of the options database file used by the `load-option` procedure. The default is `optiondb.scm` on the library path.