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Sharable SAVE file format
These files are written in EXEC using the command
@SSAVE (PAGES FROM) N (TO) N, (FROM) N (TO) N ... (ON) FILE
TOPS-20 still retains support for loading this format.
The file format consists of three sections:
- Page map, which is two pages.
- Data pages.
- Symbol table.
All pages are 512 (decimal) words.
The first word in the page map is:
| 0-17 | 18-35 |
|---|---|
| 001000 | Count of map words to follow |
Then follow count page map words:
| 0-8 | 9-17 | 18-35 |
|---|---|---|
| Access | Virtual memory page | File page |
Page numbers, both virtual and file, are normally ordered by increasing number. File pages start at 2. Pages which are unmapped (or all zero?) are not written to the map or data pages.
Access bits:
| Bit | Meaning |
|---|---|
| 0 | Write copy request - sets copy on write bit (9) |
| 2 | Read allowed |
| 3 | Write allowed |
| 4 | Execute allowed |
| 8 | Trap to user |
Bit 0 is moved to bit 9 in the access bits which is then passed to the PMAP JSYS in AC3.
Finally follows a start instruction:
| 0-35 |
|---|
| JRST to start |
The remaining space in this section is unused and should be zero.
Data pages follow and are mapped as per above. The section ends with the highest numbered file page in the map.
After the last mapped page comes a symbol table, which is alternating words of SQUOZE symbol name and 36-bit symbol value.