[Oberon] RISC5: memory instructions and flags

Joerg joerg.straube at iaeth.ch
Sat Feb 6 15:21:28 CET 2021

I have a different view:
For me the C and V flags are results of the ALU calculation as C and V need TWO operands.
But the N and Z flags can be set whenever a register is changed, as they only need ONE operand, (eg the result of a load or depositing the link address in a register)

So, I would vote to adopt the RISC documentation.


> Am 06.02.2021 um 14:53 schrieb Paul Reed <paulreed at paddedcell.com>:
> Hi Hellwig,
>> On 2021-02-06 12:46, Hellwig Geisse wrote:
>> This document doesn't say anything about setting the condition
>> flags when executing memory instructions... May the compiler rely on the fact that
>> the condition flags are set on a load?
> Sorry that this has caused you trouble. There's another gotcha here, that although the "co-optimised" version of the Verilog/Lola and the compiler allow the shorter code sequence when testing the result of a load, it doesn't work in all branch cases, because the C and V flags are *not* set.
> So you're right, if you have your own RISC implementation it might make sense to stick to the "pure" RISC ideal of only ALU ops set flags, and not also optimise the load code.
> Cheers,
> Paul
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