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// The MLAB
// --------
// In addition to Logic Array Blocks (LABs) that contain ten Adaptive Logic
// Modules (ALMs, see alm_sim.v), the Cyclone V/10GX also contain
// Memory/Logic Array Blocks (MLABs) that can act as either ten ALMs, or utilise
// the memory the ALM uses to store the look-up table data for general usage,
// producing a 32 address by 20-bit block of memory. MLABs are spread out
// around the chip, so they can be placed near where they are needed, rather than
// being comparatively limited in placement for a deep but narrow memory such as
// the M10K memory block.
//
// MLABs are used mainly for shallow but wide memories, such as CPU register
// files (which have perhaps 32 registers that are comparatively wide (16/32-bit))
// or shift registers (by using the output of the Nth bit as input for the N+1th
// bit).
//
// Oddly, instead of providing a block 32 address by 20-bit cell, Quartus asks
// synthesis tools to build MLABs out of 32 address by 1-bit cells, and tries
// to put these cells in the same MLAB during cell placement. Because of this
// a MISTRAL_MLAB cell represents one of these 32 address by 1-bit cells, and
// 20 of them represent a physical MLAB.
//
// How the MLAB works
// ------------------
// MLABs are poorly documented, so the following information is based mainly
// on the simulation model and my knowledge of how memories like these work.
// Additionally, note that the ports of MISTRAL_MLAB are the ones auto-generated
// by the Yosys `memory_bram` pass, and it doesn't make sense to me to use
// `techmap` just for the sake of renaming the cell ports.
//
// The MLAB can be initialised to any value, but unfortunately Quartus only
// allows memory initialisation from a file. Since Yosys doesn't preserve input
// file information, or write the contents of an `initial` block to a file,
// Yosys can't currently initialise the MLAB in a way Quartus will accept.
//
// The MLAB takes in data from A1DATA at the rising edge of CLK1, and if A1EN
// is high, writes it to the address in A1ADDR. A1EN can therefore be used to
// conditionally write data to the MLAB.
//
// Simultaneously, the MLAB reads data from B1ADDR, and outputs it to B1DATA,
// asynchronous to CLK1 and ignoring A1EN. If a synchronous read is needed
// then the output can be fed to embedded flops. Presently, Yosys assumes
// Quartus will pack external flops into the MLAB, but this is an assumption
// that needs testing.
// The vendor sim model outputs 'x for a very short period (a few
// combinational delta cycles) after each write. This has been omitted from
// the following model because it's very difficult to trigger this in practice
// as clock cycles will be much longer than any potential blip of 'x, so the
// model can be treated as always returning a defined result.
module MISTRAL_MLAB(input [4:0] A1ADDR, input A1DATA, A1EN, CLK1, input [4:0] B1ADDR, output B1DATA);
reg [31:0] mem = 32'b0;
always @(posedge CLK1)
if (A1EN) mem[A1ADDR] <= A1DATA;
assign B1DATA = mem[B1ADDR];
endmodule
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