> Using `lea` […] is useful if both of the operands are still needed later on in other calculations (as it leaves them unchanged)

As well as making it possible to preserve the values of both operands, it’s also occasionally useful to use `lea` instead of `add` because it preserves the CPU flags.

Loving this series! I'm currently implementing a z80 emulator (gameboy) and it's my first real introduction to CISC, and is really pushing my assembly / machine code skills - so having these blog posts coming from the "other direction" are really interesting and give me some good context.

I've implemented toy languages and bytecode compilers/vms before but seeing it from a professional perspective is just fascinating.

That being said it was totally unexpected to find out we can use "addresses" for addition on x86.

This guy is tricking us into learning assembly! Get 'em!!

Honestly, x86 is not nearly as CISC as those go. It just has a somewhat developed addressing modes comparing to the utterly anemic "register plus constant offset" one, and you are allowed to fold some load-arithmetic-store combinations into a single instruction. But that's it, no double- or triple-indexing or anything like what VAXen had.

    BINOP   disp(rd1+rd2 shl #N), rs

        vs.

    SHL     rTMP1, rd2, #N
    ADD     rTMP1, rTMP1, rd1
    LOAD    rTMP2, disp(rTMP1)
    BINOP   rTMP2, rTMP2, rs
    STORE   disp(rTMP1), rTMP2

And all it really takes to support this is just adding a second (smaller) ALU on your chip to do addressing calculations.