Auxiliary (Secondary) Memory
Auxiliary memory is non-volatile storage that is not directly addressable by the CPU. Its contents must first be transferred into main memory (usually by DMA) before they can be used.
Key properties:
- NON-VOLATILE (survives power-off)
- Very large capacity
- Very low cost per bit
- Much slower than main memory
- Accessed in BLOCKS (sectors), never in single bytes
1. Magnetic Disk
Physical structure:
PLATTER : a rigid circular disk coated with magnetic material
SURFACE : each platter has two usable surfaces
TRACK : one concentric circle on a surface
SECTOR : an arc of a track — the smallest addressable unit
(traditionally 512 bytes, now often 4096 bytes)
CYLINDER : the set of tracks at the same radius on ALL surfaces
HEAD : one read/write head per surface, all mounted on a
single moving ARM ASSEMBLY
Top view of one surface Side view of a disk pack
.---------------. ==== head
/ .---------. \ ---------------- surface 0
| / .---. \ | ---------------- surface 1
| | | hub | | | <- tracks ---------------- surface 2
| \ '---' / | ---------------- surface 3
\ '---------' /
'---------------'
sectors radiate outward
Disk capacity calculation — the standard numerical
Capacity = surfaces x tracks/surface x sectors/track x bytes/sector
Q: A disk has 4 platters, 1024 tracks per surface, 128 sectors per
track, and 512 bytes per sector. What is the total capacity?
Surfaces = 4 platters x 2 = 8
Capacity = 8 x 1024 x 128 x 512 bytes
= 8 x 1024 x 65,536
= 536,870,912 bytes
= 512 MB
Disk access time — the second standard numerical
Total access time = SEEK TIME + ROTATIONAL LATENCY + TRANSFER TIME
SEEK TIME : move the arm to the correct cylinder (mechanical,
the largest and most variable component)
ROTATIONAL LATENCY : wait for the required sector to rotate under the head
AVERAGE = half a revolution = (60 / RPM) / 2
TRANSFER TIME : time to read the sector as it passes under the head
= sector size / transfer rate
Q: A disk rotates at 7200 RPM, has an average seek time of 8 ms,
512-byte sectors and 128 sectors per track. Find the average
time to read one sector.
Time per revolution = 60 / 7200 s = 8.33 ms
Average rotational latency = 8.33 / 2 = 4.17 ms
Transfer time for one sector = 8.33 ms / 128 = 0.065 ms
Total = 8 + 4.17 + 0.065 = 12.24 ms
Q: What is the data transfer rate of the disk above?
One track = 128 x 512 = 65,536 bytes, read in one revolution (8.33 ms)
Rate = 65,536 / 0.00833 = 7,867,000 bytes/s ≈ 7.5 MB/s
Disk addressing
CHS addressing (older): Cylinder, Head, Sector
LBA addressing (modern): Logical Block Address — a single linear
number; the drive translates it internally.
Conversion:
LBA = (C x heads_per_cylinder + H) x sectors_per_track + (S - 1)
2. Magnetic Tape
A long plastic strip coated with magnetic material, wound on reels.
Structure: data is written in parallel TRACKS across the tape width
(typically 9 tracks: 8 data bits + 1 parity bit),
organised into RECORDS separated by INTER-RECORD GAPS (IRG).
ACCESS METHOD: strictly SEQUENTIAL. To read record 500 you must
pass over records 1 to 499.
| Property | Value |
|---|
| Access | Sequential only |
| Access time | Seconds to minutes |
| Capacity | Very high (LTO-9: 18 TB native) |
| Cost per GB | Lowest of all media |
| Durability | 30 years archival |
| Use | Backup, archival, cold storage |
Tape capacity numerical:
Q: A tape has a density of 1600 bytes per inch, records of 800 bytes,
and an inter-record gap of 0.5 inch. How much tape does one record
occupy, and what is the storage efficiency?
Record length on tape = 800 / 1600 = 0.5 inch
Total per record = 0.5 + 0.5 = 1.0 inch
Efficiency = 0.5 / 1.0 = 50 %
(Blocking several records together between gaps raises this
efficiency dramatically — this is why tapes use large blocks.)
3. Optical Storage
| Medium | Capacity | Wavelength | Notes |
|---|
| CD-ROM | 700 MB | 780 nm (infrared) | Data in a single spiral track |
| DVD | 4.7 GB (SL), 8.5 GB (DL) | 650 nm (red) | Smaller pits, tighter tracks |
| Blu-ray | 25 GB (SL), 50 GB (DL) | 405 nm (blue-violet) | Shortest wavelength → densest pits |
How optical discs work:
Data is stored as PITS and LANDS along a spiral track.
A laser is reflected off the surface; the transition between a pit
and a land scatters the light and is read as a 1; no transition = 0.
Variants: -ROM (read only), -R (write once), -RW (rewritable,
uses a phase-change material)
4. Solid State Drives (SSD)
Built from NAND FLASH memory — floating-gate transistors that hold
charge without power.
NO moving parts:
+ no seek time, no rotational latency
+ shock resistant, silent, low power
- limited write endurance (each cell survives 1,000 - 100,000
program/erase cycles)
- erases happen in large BLOCKS, writes in smaller PAGES
-> WRITE AMPLIFICATION
- needs WEAR LEVELLING (spread writes evenly across all cells)
and GARBAGE COLLECTION, managed by the drive's controller
| Cell type | Bits per cell | Endurance | Cost |
|---|
| SLC | 1 | ~100,000 cycles | Highest |
| MLC | 2 | ~10,000 | High |
| TLC | 3 | ~3,000 | Medium |
| QLC | 4 | ~1,000 | Lowest |
5. HDD vs SSD
| Basis | Hard disk (HDD) | Solid state (SSD) |
|---|
| Technology | Magnetic, rotating platters | NAND flash, no moving parts |
| Access time | 5–10 ms | 50–100 µs |
| Random access | Slow (seek + latency) | Fast and uniform |
| Sequential rate | 100–250 MB/s | 500 MB/s – 7 GB/s (NVMe) |
| Noise / power | Audible, higher | Silent, lower |
| Shock resistance | Poor | Excellent |
| Cost per GB | Low | Higher |
| Endurance | Unlimited writes (mechanical wear instead) | Limited write cycles |
| Best for | Bulk/archival storage | OS, applications, databases |
6. RAID (bonus — commonly asked)
RAID = Redundant Array of Independent Disks
RAID 0 : STRIPING — data split across disks. Fast, NO redundancy.
Capacity = n x disk; one failure loses everything.
RAID 1 : MIRRORING — identical copies. Capacity = disk (50%).
Survives one failure per mirror pair.
RAID 5 : STRIPING + DISTRIBUTED PARITY — survives ONE disk failure.
Capacity = (n - 1) x disk.
RAID 6 : DOUBLE PARITY — survives TWO simultaneous failures.
Capacity = (n - 2) x disk.
RAID 10: MIRROR of STRIPES — fast and redundant, 50% capacity.
Summary
Auxiliary memory : non-volatile, block-accessed, not CPU-addressable
Disk capacity = surfaces x tracks x sectors x bytes/sector
Disk access time = seek + rotational latency + transfer
Avg rotational latency = (60 / RPM) / 2
Tape : sequential only, cheapest per GB, archival
SSD : no mechanical delay, limited write endurance
Next: the one memory in the hierarchy that is searched by content rather than by address.
