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Memory and Remanence

The exam recently added timely topics such as remanence properties of Solid State Drives (SSDs), discussed shortly. We will begin by discussing computer memory itself, followed by remanence properties of volatile and non-volatile memory. Note that related concepts such as memory protection and CPU design are described in Chapter 4, Domain 3: Security Architecture and Engineering.

Data Remanence

The term data remanence is important to understand when discussing media sanitization and data destruction. Data remanence is data that persists beyond non-invasive means to delete it. Though data remanence is sometimes used specifically to refer to residual data that persists on magnetic storage, remanence concerns go beyond just that of magnetic storage media. Security professionals must understand the remanence properties of various types of memory and storage, and appreciate the steps to make data unrecoverable.

Memory

Memory is a series of on-off switches representing bits: 0s (off) and 1s (on). Memory may be chip-based, disk-based, or use other media such as tape. RAM is Random Access Memory: “random” means the CPU may randomly access (jump to) any location in memory. Sequential memory (such as tape) must sequentially read memory, beginning at offset zero, to the desired portion of memory. Volatile memory (such as RAM) loses integrity after a power loss; non-volatile memory (such as ROM, disk, or tape) maintains integrity without power.

Real (or primary) memory, such as RAM, is directly accessible by the CPU and is used to hold instructions and data for currently executing processes. Secondary memory, such as disk-based memory, is not directly accessible.

Cache Memory

Cache memory is the fastest memory on the system, required to keep up with the CPU as it fetches and executes instructions. The data most frequently used by the CPU is stored in cache memory. The fastest portion of the CPU cache is the register file, which contains multiple registers. Registers are small storage locations used by the CPU to store instructions and data.

The next fastest form of cache memory is Level 1 cache, located on the CPU itself. Finally, Level 2 cache is connected to (but outside) the CPU. SRAM (Static Random Access Memory) is used for cache memory.

Note
The memory closest to the CPU (cache memory) is the fastest and most expensive memory in a computer. As you move away from the CPU, from SRAM, to DRAM, to disk, to tape, etc., the memory becomes slower and less expensive.