PiDP-8/I Software

RALF is the back-end assembler for OS/8's FORTRAN IV compiler. As a result of that support role, it has a few primary advantages over PAL8:

*   relocatable output code
*   easy access to the floating-point processor (FPP) peripherals for the PDP-8 family

The RALF assembler has a mode switch that causes it to generate absolute-addressed code, as PAL8 does. In this mode, it is called FLAP.

RALF has the following advantages over FLAP:
The FPP modes are the key advantage of RALF/FLAP. Vincent Slyngstad [suggests][smal8] that you think of these as assemblers for the FPP rather than for the PDP-8.

*   Relocatable code

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.

RALF and FLAP share many advantages over the PAL family assemblers.

The original FPP peripheral was the [FPP-12][fpp12]. Although it was introduced with the PDP-12, it is not restricted to the PDP-12: it also works with all of the other PDP-8 family computers contemporaneous with and preceding it, going back to the original PDP-8, because it is a bus peripheral rather than a feature of the CPU proper. There was also a follow-on peripheral for the PDP-8/a called the [FPP8-A][fpp8a], which is compatible with the FPP-12. The FPP features of RALF/FLAP facilitate use of these peripherals in programs you write in those assembly languages.
The main advantage is that these assemblers have direct support for the PDP-8 family Floating Point Processor peripherals. The first such peripheral was the [FPP-12][fpp12]. Although it was introduced with the PDP-12, it is not restricted to the PDP-12: it also works with all of the other PDP-8 family computers contemporaneous with and preceding it, going back to the original PDP-8, because it is a bus peripheral rather than a feature of the CPU proper. There was also a follow-on peripheral for the PDP-8/a called the [FPP8-A][fpp8a], which is compatible with the FPP-12. The FPP features of RALF/FLAP facilitate use of these peripherals in programs you write in those assembly languages. the FPP modes. Vincent Slyngstad [suggests][smal8] that you think of RALF and FLAP as assemblers for the FPP rather than for the PDP-8.

Although the basic statement syntax in RALF and FLAP is similar to that of the PAL family, these assemblers differ from PAL family assemblers in many ways:

*   Expressions:
    *   Multiplication is `*` rather than `^`, achieved by using the `ORG` pseudo-op to set the location counter
    *   Division is `/` rather than `%`, achieved by requiring whitespace before `/` used to start a comment and disallowing spaces in expressions
    *   There is no Boolean `AND` operator `&`
*   Added FPP instructions and FPP-related psuedo-ops:
    *   Arithmetic: `FADD`, `FADDM`, `FDIV`, `FLDA`, `FMUL`, `FMULM`, `FSTA`, `FSUB`
    *   Conditional jumps: `JEQ`, `JGE`, `JLE`, `JA`, `JNE`, `JGT`, `JAL`
    *   FPP to PDP-8 jumps: `JXN`, `TRAP3`, `TRAP4`
    *   Indexing: `ADDX`, `INDEX, `LDX`
    *   No-op: `NOP`
    *   Pointer moves: `JSA`, `JSR`, `SETB`, `SETX`
    *   Processor Operations: `FCLA`, `FEXIT`, `FNEG`, `FNORM`, `FPAUSE`, `JAC`, `STARTD`, `STARTF`
    *   Register manipulation: `ALN`, `ATX`, `XTA`
    *   Require double-precision hardware: `DPCHK`
*   Use 15-bit addresses directly in FPP instructions, rather than the [PDP-8's memory model][mm8]; `BASE` psuedo-op
*   Many new and changed pseudo-ops:
    *   `LISTOF` and `LISTON` to stop and start listing output rather than the PAL family's `XLIST` pseudo-op
    *   Conditional assembly additions relative to PAL8: `IFPOS`, `IFNEG`, `IFSW`, `IFNSW`, `IFFLAP`, `IFRALF`
    *   `IFREF` replaces `IFDEF` pseudo-op, having a superset of its functionality
    *   `REPEAT` pseudo-op to assemble the following line multiple times
    *   More ways to define constants of various sorts: , `ADDR`, `E`, `F`, `S`
    *   Pseudo-ops to support FORTRAN IV features: `COMMON`, `COMMZ`, `ENTRY`, `EXTERN`, `FIELD1`, `SECT`, `SECT8`
        *    `S n` — single-word constant with value `n` (FLAP only)
        *    `F n` — 3-word single FP constant with value `n`
        *    `E n` — 6-word extended-precision FP constant with value `n`

If you see any of the added instructions listed above in a given assembly language program, it's a pretty good indicator that it's meant to be assembled by RALF or FLAP.

**INCOMPLETE**

[fpp8a]: https://archive.org/details/bitsavers_decpdp8pdpUsersManDec76_996770
[fpp12]: https://archive.org/details/bitsavers_decpdp12DE_2606247