Basic Flash Operations

The three low-level operations

There are three low-level operations that a flash chip supports. The read command is used to read a page from the flash; erase and program are used in tandem to write. The details:

• Read (a page): A client of the flash chip can read any page (e.g., 2KB or 4KB), simply by specifying the read command and appropriate page number to the device. This operation is typically quite fast, 10s of microseconds or so, regardless of location on the device, and (more or less) regardless of the location of the previous request (quite unlike a disk). Being able to access any location uniformly quickly means the device is a random access device.

• Erase (a block): Before writing to a page within a flash, the nature of the device requires that you first erase the entire block the page lies within. Erase, importantly, destroys the contents of the block (by setting each bit to the value 1). Therefore, you must be sure that any data you care about in the block has been copied elsewhere (to memory, or perhaps to another flash block) before executing the erase. The erase command is quite expensive, taking a few milliseconds to complete. Once finished, the entire block is reset and each page is ready to be programmed.

• Program (a page): Once a block has been erased, the program command can be used to change some of the 1’s within a page to 0’s, and write the desired contents of a page to the flash. Programming a page is less expensive than erasing a block, but more costly than reading a page, usually taking around 100s of microseconds on modern flash chips.

One way to think about flash chips is that each page has a state associated with it. Pages start in an INVALID state. By erasing the block that a page resides within, you set the state of the page (and all pages within that block) to ERASED, which resets the content of each page in the block but also (importantly) makes them programmable. When you program a page, its state changes to VALID, meaning its contents have been set and can be read. Reads do not affect these states (although you should only read from pages that have been programmed). Once a page has been programmed, the only way to change its contents is to erase the entire block within which the page resides. Here is an example of states transition after various erase and program operations within a 4-page block: