The definitions we have discussed to far have referenced character strings in core. Another kind of definition is stored on disk. The phrase
1017 SHEET FIT
declare FIT as a sheet with disk address 1017. When FIT is encountered, the scanner is directed to disk for further input.
FIT identifies a region of disk that contains source language. We call it a sheet because typically such definitions consist of about 50 lines of 40 characters, which fit nicely on a sheet of paper.
An application typically uses several sheets to describe the dictionary entries it requires. In effect, referencing a sheet moves its entries from disk to core, compiling them on the way.
A sheet may reference other sheets and thus direct the retrieval of information. For example, to generate FORTH we access ROOT, a sheet that acts like an index: it declares a number of other sheets. One of these sheets is SYSGEN, which references other sheets in a particular order and thereby assembles the system dictionary.
Sheets may be edited and definitions added or changed using definitions on the set EDIT. Each user is assigned disk space and provided tools to maintain it.
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