How NAND Flash Works Inside Your Memory Card
- scarlettkim7
- 5 days ago
- 2 min read
Updated: 4 days ago
NAND Flash Internal Operation (Easy Explanation)
NAND flash memory follows a few fundamental rules when handling data.
1. NAND flash reads and writes data only in page units.
o Typical page size: 4 KB to 16 KB
2. Once data is written, it cannot be overwritten directly.
o The existing data must be erased before new data can be written.
3. Erase operations are performed only in block units.
o Typical block size: several megabytes
Example: Internal Structure of a Modern 64 GB SD Card
Although the exact configuration varies by card, a typical modern 64 GB SD card may
look like this internally:
The Fundamental Problem: NAND Flash Cannot Overwrite Data
The key limitation of NAND flash is that data cannot be overwritten in place.
For example, assume we want to update just 1 byte inside Page p3 of Block b2.
Even though only 1 byte changes:
The smallest writable unit is a page (16 KB)
The smallest erasable unit is a block (4 MB)
As a result, the flash controller must internally perform the following steps:
1. Read the entire block b2 (4 MB)
2. Erase the entire block b2
3. Modify Page p3 with the new data
4. Write the entire block b2 (4 MB) back to flash
In other words, updating 1 byte requires reading, erasing, and rewriting 4 MB of data.
Why Early SD Cards Were Slow
Before UHS-I SD cards became common in the early 2010s, many SD cards operated in this manner.
As a result:
Small updates triggered large internal operations
Sustained write performance was poor
Even Class 10 (10 MB/s) was difficult to maintain reliably
Why Modern SD Cards Are Much Faster
Starting in the early 2010s, modern SD cards adopted several key technologies:
Dynamic Mapping
Over-Provisioning
Garbage Collection (GC)
The core idea behind these technologies is separating logical (user) addresses from
physical NAND locations.
Today, when the host requests:
“Write new data to Page p3 in Block b2”
The controller instead:
Writes the data to a different, already-erased page
Updates the internal mapping so that this new page appears as b2:p3 to the host
This approach:
Avoids immediate erase operations
Eliminates write stalls
Enables high and stable sustained write performance
Related Technical Deep Dive
For a deeper explanation of these internal memory card management technologies, read our companion article.
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