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Equation 1 · Part 3 · Comparing the Main Approaches to AI Memory Systems and the Bandwidth Wall

Symbol f_eff

B  ≈  W⋅feff,B \;\approx\; W \cdot f_{\mathrm{eff}},
fefff_{\mathrm{eff}}

What this part means

the effective transfer rate per pin.

Its job in the formula

fef_eff is a part of this expression. Its role is fixed by the surrounding article and by the operations shown in the formula.

Where the article explains it

with W the bus width in bits and fefff_{\mathrm{eff}} the effective transfer rate per pin.

The passage around this formula

…a hidden physical driver that explains why these four approaches look so different from one another: pin count. A memory bus’s raw bandwidth is well approximated by B  ≈  W⋅feffB \;\approx\; W \cdot f_{\mathrm{eff}}. with W the bus width in bits and fefff_{\mathrm{eff}} the effective transfer rate per pin. For a conventional DIMM interface, W is set by the number of package pins a processor can spare and a printed-circuit board can route, and that number has a hard physical ceiling — DDR5 alone specifies 288 pins per channel, and adding channels…

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A subscript is a label attached below a symbol. It often selects a time step, component, category, or member of a sequence.

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Sources cited in the surrounding passage

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