41C ROMs: LBL linkages

[hth]

(09-05-2019 12:10 AM)RobertM Wrote:  So basically, the first label in the UCODE block has the link setup to include the number of nulls generated (1 - number of bytes in first register) and then +3 for the "next" next link in the chain, which it can assume is right justified in a register (because it was the old .END. before the COPY). And the linkages don't matter while they exist in ROM.

As you pointed out, those engineers were extremely clever in their design solutions.

Thanks for all the help!

-Robert

I would change that 1 to a 7.

In my RPN compiler tool I start with (3 + nullsInFirstReg) as initial offset for global chain in my tools. Then I increase it by instruction size if it is not a global chain instruction. If it is a global chain, I set the offset to its instruction size and carry on. Some source maybe explains it better (or not):

Code:

buildChain :: [Node RpnInstr Context] -> (Int, [Node RpnInstr Context])
buildChain prog =
    let (_, prog') = mapAccumL chain (3 + nullsInFirstReg) prog where
            chain offset node@(INode rpn ctx) =
                let chained = (size, INode (RpnChain rpn [byte0, byte1]) ctx)
                    size = sizeof rpn
                    (regs, bytes) = offset `divMod` 7
                    packedDistance = (bytes `shiftL` 9) .|. regs
                    byte0 = fromIntegral $ 192 .|. (packedDistance `shiftR` 8)
                    byte1 = fromIntegral $ packedDistance .&. 0xff
                in case rpn of
                     END -> chained
                     LBL (OpAlpha _) -> chained
                     otherwise -> (offset + size, node)
        nullsInFirstReg = 7 - bytesInFirstReg
        bytesInFirstReg = snd $ registerCount progSize
        progSize = sum $ map (sizeof . rpnInNode) prog
    in (progSize, prog')

I should add that this is based on studying the source code of COPY and it was a while back. I did a restudy a few days ago to answer in this thread. I have not looked at how existing native MCODE RPN loaders (there were more than one, I even wrote one a long time ago) do it, as it may pose a problem to them when the first global alpha label is not first. I mean, it feels somewhat non-trivial to do this in MCODE, at least to me.

But otherwise it makes perfect sense. It also explains those annoying single superfluous END instructions inserted and it makes the COPY routine surprisingly simple. It is really very clever.