From 3443356e693af2a8cc70d504f55756d2cbdaebd1 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Nicol=C3=A1s=20Ortega=20Froysa?= <nicolas@ortegas.org>
Date: Tue, 18 Jan 2022 15:51:18 +0100
Subject: [PATCH] Add notes on memory.

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 1/FH/notes/unit-02.md | 34 ++++++++++++++++++++++++++++++++++
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 create mode 100644 1/FH/notes/unit-02.md

diff --git a/1/FH/notes/unit-02.md b/1/FH/notes/unit-02.md
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+# Unit 2
+
+## Cáculos Con Memoria
+
+### Cálculo de canal
+
+DDRX-YYYY where X is the type of DDR being used, and YYYY the frequency. It can
+also be expressed as PC(X)-ZZZZ where ZZZZ is the máximum theoretical
+transfer rate (MTTR) of the memory. To calculate the ZZZZ value from YYYY
+you must do `YYYY * 64 / 8 = ZZZZ MB/s`. To calculate the external
+frequency you must divide the YYYY value by 2, for the two bits, and for
+internal you must divide the YYYY value by the prefetch value, which is:
+
+- DDR: 2
+- DDR2: 4
+- DDR3: 8
+
+### Cálculo de Latencia
+
+Para calcular el tiempo ocioso de un CPU cuando accede a memoria, se
+calcula usando la frecuencia externa. Con esto se ha de convertir de
+frecuencia en (nano) segundos, de ahí se ha de multiplicar por el número de
+ciclos de latencia (CL), y finalmente multiplicar este número por la frecuencia
+de reloj de la CPU.
+
+E.g.
+
+```calc
+CL=5; CPU Freq=2.2GHz; DDR-800
+800/2 = 400 MHz
+1/400 = 2.5 ns
+2.5 * 5 = 12.5 ns
+12.5 * 2.2 GHz = 28 cycles
+```