RRB JE Junior Engineer · Computer Knowledge
Memory & Storage
Primary and secondary memory, RAM, ROM, cache and storage units.
Seven concepts. Memory questions almost always sort RAM versus ROM, SRAM versus DRAM, or the speed hierarchy — classifications and unit facts. Prefer tables and pegs; the GB→MB conversion is single-step arithmetic, not a multi-row ledger.
- RRB JE Junior Engineer
- Medium level
- 7 concepts
- 40 practice questions
1Primary memory: RAM and ROM
Primary memory is directly accessible by the CPU. The two names that dominate papers are RAM and ROM. RAM is volatile and read/write — it needs power to hold data and can be written during normal use. ROM is non-volatile and mostly read-only — it keeps its contents without power and is used for firmware such as the BIOS.
A memory aid from the notes: the "W" in RAM's read/Write reminds you it can be written; ROM is Read-only and non-volatile.
Figure. RAM is read/write and loses data when power drops; ROM keeps firmware across power-off.
| Memory | Volatility | Read / write | Typical role |
|---|---|---|---|
| RAM | Volatile (needs power) | Read/write | Working main memory |
| ROM | Non-volatile | Mostly read-only | BIOS / firmware |
You switch a PC off at the wall. Which primary memory loses its contents, and which retains firmware without power?
- ROM loses data; RAM retains the BIOS
- RAM loses data (volatile); ROM retains its contents (non-volatile)
- Both lose data because primary memory is always volatile
RAM is volatile and clears without power. ROM is non-volatile and keeps firmware. Primary memory as a class includes both; it is not "always volatile".
2SRAM versus DRAM
RAM itself splits into two common types in exam notes. SRAM (Static RAM) uses flip-flops, is fast and expensive, and is used in cache. DRAM (Dynamic RAM) uses capacitors, is cheaper, needs refreshing, and is used as main memory.
Static means no refresh; dynamic means refresh required. Cache → SRAM; main RAM modules → DRAM.
Figure. SRAM is faster and used in cache; DRAM is denser main memory and must be refreshed.
| Type | Technology cue | Speed / cost | Where used |
|---|---|---|---|
| SRAM | Flip-flops; no refresh | Faster, more expensive | Cache |
| DRAM | Capacitors; needs refresh | Cheaper, slower than SRAM | Main memory |
A board question says a memory cell stores bits on capacitors and must be refreshed periodically, and that this cheap technology builds the computer's main working memory. Which RAM type is that?
- SRAM — used in cache because capacitors are fastest
- DRAM — capacitors, refresh, main memory
- ROM — because refresh makes it non-volatile
Capacitors plus refresh plus main memory is DRAM. SRAM uses flip-flops, needs no refresh, and builds cache. Refresh does not make DRAM into ROM.
3ROM variants: PROM, EPROM, EEPROM
ROM is not only factory-sealed "mask ROM" in exam lists. PROM is programmable once. EPROM is erasable by ultraviolet light. EEPROM is electrically erasable. These variants store BIOS/firmware that must persist without power.
Match the erase method to the name: UV → EPROM; electrical → EEPROM; once-only → PROM.
Figure. PROM is programmed once; EPROM needs UV to erase; EEPROM erases electrically.
| Type | Programming / erase cue | Role cue |
|---|---|---|
| PROM | Programmable once | Firmware that is written once |
| EPROM | Erasable by UV light | Reusable after UV erase |
| EEPROM | Electrically erasable | Erase/rewrite electrically; BIOS/firmware cue |
A technician must erase a firmware chip with ultraviolet light before reprogramming it. Which ROM variant is the stem describing?
- PROM — programmable once, so UV erase is normal
- EPROM — erasable by UV light
- EEPROM — UV is the electrical erase method
UV erase names EPROM. PROM is once-only without that UV story; EEPROM is electrically erasable, not UV-erased.
4Cache and the memory hierarchy
Cache memory sits between the CPU and RAM to speed access. Within cache, levels are ordered L1 > L2 > L3 by closeness/speed in the usual exam ladder. The full speed order in the notes is Registers > Cache > RAM > SSD/HDD.
Closer to the CPU means faster, smaller, and costlier per bit. Access time follows the same idea: cache is faster than RAM, and RAM is faster than HDD among the three often compared.
Figure. Closer to the CPU means faster and smaller: registers, cache, RAM, then secondary storage.
| Level | Place in hierarchy | Cue |
|---|---|---|
| Registers | Inside CPU | Fastest, smallest |
| Cache (L1 > L2 > L3) | Between CPU and RAM | Speeds access to RAM |
| Primary (RAM) | Main working memory | Volatile read/write store |
| Secondary (SSD/HDD) | Permanent storage | Slower; keeps data without power |
A question asks for the fastest storage among registers, cache, RAM and HDD, and separately asks which sits between CPU and RAM to reduce wait time. Which pair is correct?
- HDD fastest; RAM sits between CPU and cache
- Registers fastest; cache sits between CPU and RAM
- RAM fastest; registers sit below HDD on the speed ladder
Registers top the speed order; cache sits between CPU and RAM. HDD is among the slowest of those; RAM is not above registers.
5Secondary storage
Secondary storage is non-volatile and permanent: HDD, SSD, USB drive, CD/DVD. It is slower than primary memory but retains data without power — that retention is why files survive a shutdown.
When a stem contrasts "working memory" with "permanent store", primary (RAM) versus secondary (disk/USB/optical) is the cut.
Figure. Secondary storage keeps data without power: hard disk, SSD flash, optical discs.
| Medium | Volatility | Role cue |
|---|---|---|
| HDD | Non-volatile | Magnetic disk; large capacity, slower than SSD |
| SSD | Non-volatile | Faster secondary store than HDD in the hierarchy notes |
| USB drive | Non-volatile | Removable flash storage |
| CD / DVD | Non-volatile | Optical discs |
After a clean shutdown, a student's project file is still on the machine the next morning, but the unsaved document that lived only in working memory is gone. Which store kept the file, and which lost the unsaved data?
- RAM kept the file; SSD/HDD lost the unsaved document
- Secondary storage (e.g. SSD/HDD) kept the file; RAM lost the unsaved document
- ROM kept the unsaved document; cache kept the file
Saved files persist on secondary storage. Unsaved working data in volatile RAM vanishes without power. ROM holds firmware, not the student's document; cache is also volatile working store.
6Volatile versus non-volatile memory
Volatile memory loses data when power is cut — RAM and cache are the volatile pair in the notes. Non-volatile memory retains data without power — ROM, HDD and SSD sit on that side.
Volatile ≈ "vanishes" when power goes. Do not call all primary memory volatile: ROM is primary yet non-volatile.
Figure. Volatile RAM clears on power-off; non-volatile media keep their contents.
| Class | On power-off | Examples in notes |
|---|---|---|
| Volatile | Loses data | RAM, cache |
| Non-volatile | Retains data | ROM, HDD, SSD |
Which pair is entirely volatile, so both lose contents on power-off?
- ROM and SSD
- RAM and cache
- HDD and ROM
RAM and cache are the volatile examples in the notes. ROM, HDD and SSD are non-volatile.
7Bits, bytes and the binary capacity ladder
Capacity questions use the binary ladder from the notes: 1 byte = 8 bits; 1 KB = 1024 B; 1 MB = 1024 KB; 1 GB = 1024 MB; 1 TB = 1024 GB. The trap is multiplying by 1000 instead of 1024 when the paper assumes classic computing units.
Cost per bit runs opposite to capacity on the hierarchy: registers and cache cost most per bit; HDD costs least per bit among the usual comparison set.
Figure. Binary capacity steps by 1024: B to KB to MB to GB to TB.
| Unit | Equals |
|---|---|
| Bit | Smallest unit (0 or 1) |
| Byte | 8 bits |
| KB | 1024 bytes |
| MB | 1024 KB |
| GB | 1024 MB |
| TB | 1024 GB |
A pen drive is labelled 16 GB and the question states binary units. How many megabytes is that?
- 16000 MB, using ×1000
- 16384 MB, because 16 \times 1024 = 16384
- 16 MB, because GB and MB are interchangeable labels
Under the notes' ladder, 1 GB = 1024 MB, so 16 GB = 16384 MB. 16000 is the decimal-thousand trap; 16 MB confuses the unit names.
Notes
- Primary memory is directly accessible by the CPU: RAM (volatile, read/write) and ROM (non-volatile, mostly read-only).
- RAM types: SRAM uses flip-flops and is fast/expensive (used in cache); DRAM uses capacitors, is cheaper and needs refreshing (used as main memory).
- ROM variants: PROM (programmable once), EPROM (erasable by UV light), EEPROM (electrically erasable) — used to store the BIOS/firmware.
- Cache memory sits between CPU and RAM to speed access; the hierarchy by speed is Registers > Cache > RAM > SSD/HDD.
- Secondary storage is non-volatile and permanent: HDD, SSD, USB drive, CD/DVD; it is slower but retains data without power.
Formulas
- 1 Byte = 8 bits; 1 KB = 1024 B; 1 MB = 1024 KB; 1 GB = 1024 MB; 1 TB = 1024 GB.
- Memory speed order: Registers > Cache (L1>L2>L3) > Primary (RAM) > Secondary (SSD/HDD).
- Volatile memory (RAM, cache) loses data on power-off; non-volatile (ROM, HDD, SSD) retains it.
- Access time: cache < RAM < HDD (cache fastest, HDD slowest of the three).
- Cost per bit is inversely related to capacity: registers/cache cost most per bit, HDD least.
Exam traps & shortcuts
- RAM = Random/Read-write + volatile; ROM = Read-only + non-volatile — the 'W' in RAM's read/Write reminds you it can be written.
- SRAM = Static (no refresh, fast, cache); DRAM = Dynamic (needs refresh, main memory).
- Closer to the CPU = faster + smaller + costlier per bit (registers/cache).
Reference tables
RAM/ROM, SRAM/DRAM, ROM variants, hierarchy, volatility and the ×1024 ladder in one place.
| Topic | Hold this |
|---|---|
| Primary | RAM volatile R/W; ROM non-volatile, mostly read-only (BIOS) |
| SRAM / DRAM | SRAM = cache, no refresh; DRAM = main memory, refresh |
| ROM variants | PROM once; EPROM UV; EEPROM electrical |
| Speed order | Registers > Cache (L1>L2>L3) > RAM > SSD/HDD |
| Volatility | Volatile: RAM, cache; Non-volatile: ROM, HDD, SSD |
| Units | ×1024 each rung; 1 byte = 8 bits |
Recap
If you keep only pegs, keep these.
- RAM
- Volatile, read/write; loses data on power-off
- ROM
- Non-volatile, mostly read-only; BIOS/firmware
- SRAM / DRAM
- SRAM = fast cache, no refresh; DRAM = main memory, needs refresh
- ROM variants
- PROM once; EPROM UV erase; EEPROM electrical erase
- Hierarchy
- Registers > Cache (L1>L2>L3) > RAM > SSD/HDD
- Closer to CPU
- Faster + smaller + costlier per bit
- Secondary
- HDD, SSD, USB, CD/DVD — non-volatile permanent store
- Volatile pair
- RAM and cache lose data without power
- Binary ladder
- KB/MB/GB/TB each ×1024; 1 byte = 8 bits
- 16 GB
- 16384 MB in binary units (16 \times 1024)
Practise Memory & Storage
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- 40 exam-style questions on this topic, with explanations
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