35 lines
996 B
Markdown
35 lines
996 B
Markdown
# Unit 2
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## Cáculos Con Memoria
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### Cálculo de canal
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DDRX-YYYY where X is the type of DDR being used, and YYYY the frequency. It can
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also be expressed as PC(X)-ZZZZ where ZZZZ is the máximum theoretical
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transfer rate (MTTR) of the memory. To calculate the ZZZZ value from YYYY
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you must do `YYYY * 64 / 8 = ZZZZ MB/s`. To calculate the external
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frequency you must divide the YYYY value by 2, for the two bits, and for
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internal you must divide the YYYY value by the prefetch value, which is:
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- DDR: 2
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- DDR2: 4
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- DDR3: 8
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### Cálculo de Latencia
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Para calcular el tiempo ocioso de un CPU cuando accede a memoria, se
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calcula usando la frecuencia externa. Con esto se ha de convertir de
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frecuencia en (nano) segundos, de ahí se ha de multiplicar por el número de
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ciclos de latencia (CL), y finalmente multiplicar este número por la frecuencia
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de reloj de la CPU.
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E.g.
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```calc
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CL=5; CPU Freq=2.2GHz; DDR-800
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800/2 = 400 MHz
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1/400 = 2.5 ns
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2.5 * 5 = 12.5 ns
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12.5 * 2.2 GHz = 28 cycles
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```
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