PiDP-8/I Software

[RALF](#ralf) was based on a simpler assembler called FLAP, which generates absolute-addressed code, just as PAL8 does. This section will simply describe the differences in FLAP relative to RALF. These differences aside, FLAP shares the feature set of RALF.

The primary advantage FLAP has over RALF is that, because it is difficult to ascribe meaning to the "page" concept in a relocatable assembler, RALF doesn't support the `PAGE` pseudo-op, while FLAP does. FLAP also supports the PAL family's zero-page and current-page literal syntaxes, while RALF does not.

FLAP has one pseudo-op that RALF does not, `S` for defining a single-word integer constant.

Because FLAP output is absolute-addressed, it writes `*.BN` files as output rather than `*.RL`. These are linked and loaded by the OS/8 absolute loader, `ABSLDR.SV`. You can link and run several files directly this way, or you can just load them into core and `SAVE` them to a `*.SV` file for later use by the OS/8 `R` command. This is altogether simpler than the RALF scheme, but it means you miss out on the power overlay loading scheme: FLAP programs cannot be any larger than core memory unless you, the programmer, do the manual work to create your own overlay swapping scheme.
Because FLAP output is absolute-addressed, it writes `*.BN` files as output rather than `*.RL`. These are linked and loaded by the OS/8 absolute loader, `ABSLDR.SV`. You can link and run several files directly this way, or you can just load them into core and `SAVE` them to a `*.SV` file for later use by the OS/8 `R` command. This is both simpler and less powerful than the RALF/FRTS dynamic overlay scheme: FLAP programs cannot be any larger than core memory unless you, the programmer, do the manual work to create your own overlay swapping scheme.

`FLAP.SV` is currently not provided on the PiDP-8/I project's OS/8 disk packs. It is not known at this time if it is because FLAP was eventually removed from the OS/8 FORTRAN IV system or if we simply missed it when curating the input media we use for building our OS/8 disk packs. When we figure that out, I'll update this article.
`FLAP.SV` is currently not provided on the PiDP-8/I project's OS/8 disk packs. It is not present on the OS/8 source media we're using, but we don't know if this is because it was removed from OS/8 by DEC or if we simply missed the FLAP source media when curating the input media we use for building our OS/8 disk packs. When we figure that out, I'll update this article.

There is a `FLAP.SAV` program on the PiDP-8/I project's TSS/8 boot media, but we have not yet spent the time to figure out how to make it work. None of the TSS/8 manuals we have access to talk about FLAP, so this is not likely to be a quick project.

By the way, you may find pages online that claim that FLAP was derived from RALF, rather than the other way around. I'm basing my claim on what DEC's own documentation says: "RALF, an extension of FLAP..." This quote was found in two different OS/8 manuals published about 5 years apart.


### <a id="sabr"></a>SABR

As [RALF](#ralf) is to OS/8's FORTRAN IV compiler, so the [Symbolic Assembler for Binary Relocatable programs](http://homepage.cs.uiowa.edu/~jones/pdp8/faqs/#langs) is to OS/8's FORTRAN II compiler.