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  • gloriouscowgloriouscow@oldbytes.space
    Oct 12, 2026, 8:41 PM

    This is going to make for an interesting article contrasting the BIU timings between 8088 and V20.

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  • gloriouscowgloriouscow@oldbytes.space
    Oct 12, 2026, 9:10 PM

    The V20 appears to have some pipelining going on, which is fun. The EU can continue in some cases while a bus request is in progress, whereas on the 8088 it would always stall.

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  • gloriouscowgloriouscow@oldbytes.space
    Oct 12, 2026, 10:26 PM

    Here's a cool optimization on the V20 I'm calling write forwarding.

    I can call it whatever I want. If you want to call microcode stuff something then decode your own microcode.

    This is from the MOVSW instruction:

    0A51: SI -> DP                    ---  EC=MR
    0A52: SIGMA -> SI   DI -> tmpB    ---  ALU ADD, tmpC
    0A53: DI -> DP                    --E  EC=MW, SR=ES

    The V20 has a register, OPR, which holds the results of bus operations. Duh. But notice here, there's no OPR. That's odd.

    What's happening is that we can pipeline the bus requests. EC=MR requests a read operation from the BIU and before it even starts EC=MW requests a write.

    So where the hell is OPR? Well the key is we don't actually need the value. MOVSW just copies it blindly.

    So as best as I can tell, if you queue a read and a write back to back the result of the read just goes right back out the write, automatically.

    write forwarding. fuck yeah.

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  • gloriouscowgloriouscow@oldbytes.space
    Oct 12, 2026, 10:28 PM

    This is the secret sauce for how the V20 keeps the bus full with string operations while the 8088 sits there peeing on its hands for 6 cycles per iteration. It's pipelining the address calculations.

    NEC you freaky.

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  • gloriouscowgloriouscow@oldbytes.space
    Oct 12, 2026, 10:49 PM

    WR typically has to calculate an address for the write and that incurs a delay.

    But if you've already resolved IND and OPR, it appears as if that resolves the pending write address and the V20 can just yeet it immediately.

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  • gloriouscowgloriouscow@oldbytes.space
    Oct 12, 2026, 10:56 PM

    the JCXZ instruction jumps if CX is 0.

    my test set for JCXZ never sets CX to 0. Whoopsie.

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  • William D. Jonescr1901@mastodon.social
    Oct 12, 2026, 10:31 PM

    @gloriouscow Similar-in-spirit trick for Sentinel's microcode: codeberg.org/cr1901/sentinel/s

    I have the current shifted value at the output of the ALU and a shift count at the input of the ALU. I send the shift value to the ALU input as the decremented shift count reaches the ALU output. I repeat this in a loop until the shift count goes to zero. That way, I'm shifting and counting the number of shifts in lockstep using a single ALU resource :D.

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