intel_alm: Documentation improvements

techlibs/intel_alm/common/alm_sim.v

@@ -1,3 +1,72 @@
+// The core logic primitive of the Cyclone V/10GX is the Adaptive Logic Module
+// (ALM). Each ALM is made up of an 8-input, 2-output look-up table, covered 
+// in this file, connected to combinational outputs, a carry chain, and four
+// D flip-flops (which are covered as MISTRAL_FF in dff_sim.v).
+//
+// The ALM is vertically symmetric, so I find it helps to think in terms of
+// half-ALMs, as that's predominantly the unit that synth_intel_alm uses.
+//
+// ALMs are quite flexible, having multiple modes.
+//
+// Normal (combinational) mode
+// ---------------------------
+// The ALM can implement:
+// - a single 6-input function (with the other inputs usable for flip-flop access)
+// - two 5-input functions that share two inputs
+// - a 5-input and a 4-input function that share one input
+// - a 5-input and a 3-or-less-input function that share no inputs
+// - two 4-or-less-input functions that share no inputs
+//
+// Normal-mode functions are represented as MISTRAL_ALUTN cells with N inputs.
+// It would be possible to represent a normal mode function as a single cell -
+// the vendor cyclone{v,10gx}_lcell_comb cell does exactly that - but I felt
+// it was more user-friendly to print out the specific function sizes
+// separately.
+//
+// With the exception of MISTRAL_ALUT6, you can think of two normal-mode cells
+// fitting inside a single ALM.
+//
+// Extended (7-input) mode
+// -----------------------
+// The ALM can also fit a 7-input function made of two 5-input functions that
+// share four inputs, multiplexed by another input.
+//
+// Because this can't accept arbitrary 7-input functions, Yosys can't handle
+// it, so it doesn't have a cell, but I would likely call it MISTRAL_ALUT7(E?)
+// if it did, and it would take up a full ALM.
+//
+// It might be possible to add an extraction pass to examine all ALUT5 cells
+// that feed into ALUT3 cells to see if they can be combined into an extended
+// ALM, but I don't think it will be worth it.
+//
+// Arithmetic mode
+// ---------------
+// In arithmetic mode, each half-ALM uses its carry chain to perform fast addition
+// of two four-input functions that share three inputs. Oddly, the result of
+// one of the functions is inverted before being added (you can see this as
+// the dot on a full-adder input of Figure 1-8 in the Handbook).
+//
+// The cell for an arithmetic-mode half-ALM is MISTRAL_ALM_ARITH. One idea
+// I've had (or rather was suggested by mwk) is that functions that feed into
+// arithmetic-mode cells could be packed directly into the arithmetic-mode
+// cell as a function, which reduces the number of ALMs needed.
+//
+// Shared arithmetic mode
+// ----------------------
+// Shared arithmetic mode looks a lot like arithmetic mode, but here the
+// output of every other four-input function goes to the input of the adder
+// the next bit along. What this means is that adding three bits together can
+// be done in an ALM, because functions can be used to implement addition that
+// then feeds into the carry chain. This means that three bits can be added per
+// ALM, as opposed to two in the arithmetic mode.
+//
+// Shared arithmetic mode doesn't currently have a cell, but I intend to add
+// it as MISTRAL_ALM_SHARED, and have it occupy a full ALM. Because it adds
+// three bits per cell, it makes addition shorter and use less ALMs, but
+// I don't know enough to tell whether it's more efficient to use shared
+// arithmetic mode to shorten the carry chain, or plain arithmetic mode with
+// the functions packed in.
+
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 (* abc9_lut=2, lib_whitebox *)