Layout of an SSD

Introduction

A Solid-state drive (SSD) is a nonvolatile electronic storage medium used in computers and laptops. An SSD is a secondary storage device that uses flash memory.

Types of ROM

Before discussing the layout of SSD, let's understand the different categories of ROM and where flash memory falls in the category.

Read-only memory (ROM) or firmware is a memory technology used when the data is written once and not changed. Computers use this storage technology when the information is stored permanently, even when the power is off. As a result, they are durable and less expensive than RAM technologies.

ROM is categorized based on the number of program/erase (P/E) cycles it can handle, which is the number of times the chip can be erased and programmed before it becomes unusable.

Types of ROM include:

• Masked Read-Only Memory (MROM): MROM is a read-only memory chip programmed during manufacturing. It cannot be erased or rewritten.

• Programmable Read-Only Memory (PROM): PROM can be programmed once by the user and cannot be erased or rewritten.

• Erasable Programmable Read-Only Memory (EPROM): EPROM devices are subject to erasure by exposing them to UV light for a few minutes.

• Electrically Erasable Programmable Read-Only Memory (EEPROM): EEPROM devices are subject to electric currents to erase and reprogram the data. Any location can be erased and reprogrammed in a matter of milliseconds. The number of times they can be erased and reprogrammed is limited.

• Flash memory: Flash memory is an extension of EEPROM, and is erased and reprogrammed in blocks, which makes it faster. The number of times it can be erased and reprogrammed is limited.

Architecture

An SSD includes two major components:

• Flash memory

• SSD controller

Flash memory

As stated above, flash memory is a type of storage technology where the data is erased in units of blocks, but is reprogrammed at the byte level.

A memory cell (charge trap memory cell) is the primary storage unit in the flash memory. A charge trap is a vital component of a memory cell that enables the storing of information by placing different levels of electrons onto a charge trap using a charge trap flash technology. Charge trap flash technology allows for multiple bits to be stored, represented in the form of electron charge in a single memory cell. Once charged, the memory cell can hold electrons for decades, allowing data durability.

A string is a group of memory cells stacked together vertically. Only one memory cell from a page is activated at any given time when information is read or written. An array is a group of strings, and a page is a group of arrays. Each page can hold a few kilobytes of data. A block is a collection of pages. A group of 64 to 512 pages forms a block. A plane is a collection of blocks, and a die is a collection of planes. A single SSD can have multiple dies.

Even though the memory cell is the storage unit in the SSD, the smallest unit that can be read and written is a page. However, only empty memory cells can be erased and reprogrammed, and the smallest erasable entity is a block. Hence, SSDs are commonly referred to as erase blocks.

Note: The three common types of SSD storage interfaces used in the market are SATA, SAS, and NVMe.