Memory Write Ability and Storage Permanence

Computer Organization & Embedded System — Computer Arithmetic and Memory System, NEC licence examination syllabus (Nepal Engineering Council).

Memory Write-Ability and Storage Permanence: two spectrums, one framework

A simple mental model (from Vahid's embedded systems textbook) for classifying ANY memory type.

Instead of memorizing every memory type separately, place each one on two independent scales:

Write-ability spectrum: how easily new data can be written — high (RAM, instantly rewritable) → medium (Flash/EEPROM, rewritable but slower/limited cycles) → low (mask-programmed ROM, fixed at manufacture, never rewritable).

Storage-permanence spectrum: whether data survives without power — nonvolatile (ROM family, Flash — keeps data with no power) → volatile (SRAM, DRAM — loses data instantly without power).

💡 Exam trick: they may give a table of memory types and ask you to place each on both spectrums — remember Flash sits in the "sweet spot" of decent write-ability AND full nonvolatility, which is exactly why it replaced older ROM types in most modern devices.

Syllabus points

Write-ability spectrum
Storage-permanence spectrum
Placing each memory type on both spectrums

Create a free account to tick topics off, take notes as you read, watch the video lessons and get a day-by-day study plan built around your exam date.