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The first unit of Class 11 Computer Science looks like definitions to learn, but exam questions test whether you understand how the parts work together: why a cache exists, why ROM holds the boot loader, why you cannot install software on a computer with no storage. This post covers the unit in that order, with memory conversions worked out and the mistakes that cost marks.
What does Unit 1 cover?
The CBSE Computer Science curriculum for 2026-27 gives Unit 1, Computer Systems and Organisation, 10 of the 70 theory marks. Its first two topics are basic computer organisation (hardware, software, input and output devices, CPU, primary, cache and secondary memory, and units of memory from bit to PB) and types of software. The unit also covers operating systems, Boolean logic, number systems and encoding schemes. Our posts on Boolean logic and number systems cover those. The textbook chapter is NCERT's Computer System, Chapter 1 of the Class 11 book.
What are the main parts of a computer?
NCERT describes a computer as input devices, a CPU, memory and output devices. The CPU does the processing. Input devices such as a keyboard or scanner bring data in; output devices such as a monitor or printer send results out. All of it is connected by buses, which are physical wires:
The data bus carries data and is bidirectional, since the CPU both reads and writes.
The address bus carries the address of the memory location the CPU wants, and is unidirectional.
The control bus carries read and write signals, and is unidirectional.
Together they form the system bus.
How is memory organised?
Think of it as a ladder from fast and small to slow and large.
Cache sits between the CPU and RAM. NCERT explains that it stores copies of data from frequently accessed memory locations, so the CPU checks the cache first and only goes to RAM if the data is not there.
RAM (main memory) holds programs and data while they run. It is volatile: its contents are lost when power is switched off.
ROM is non-volatile and rarely changed. NCERT's example is the boot loader, the startup program that loads the operating system into memory.
Secondary memory (hard disk, SSD, pen drive, CD/DVD) is non-volatile, larger, slower and cheaper. The CPU cannot read it directly: data must first be brought into main memory.
That last point answers a question NCERT asks: a computer with RAM but no secondary storage has nowhere to keep installed software once the power goes off.
How do you convert between memory units?
NCERT's Table 1.1 uses powers of 1024: 1 byte = 8 bits (and 4 bits make a nibble), 1 KB = 1024 bytes, 1 MB = 1024 KB, 1 GB = 1024 MB, 1 TB = 1024 GB, 1 PB = 1024 TB. Use these in board answers.
Worked example 1. How many bytes and bits are in 2 MB?
2 MB = 2 × 1024 KB = 2048 KB.
2048 KB = 2048 × 1024 bytes = 2,097,152 bytes.
2,097,152 × 8 = 16,777,216 bits.
Worked example 2. How many 4 MB photos fit on a 16 GB pen drive, ignoring space used by the file system?
16 GB = 16 × 1024 MB = 16,384 MB.
16,384 ÷ 4 = 4096 photos.
Worked example 3. Why might a 500 GB drive show up as about 465 GB? The international standard explained by NIST reserves "giga" for 10^9, and uses "gibi" (GiB) for 2^30 = 1,073,741,824 bytes. If a drive holds 500 × 10^9 bytes and the computer divides by 2^30, it reports 500,000,000,000 ÷ 1,073,741,824 ≈ 465.66. Nothing is missing; two definitions of GB are being mixed. For CBSE, follow NCERT's 1024 convention unless the question says otherwise.
What are the types of software?
NCERT groups software into three kinds:
System software works directly with the hardware: the operating system, system utilities (such as disk formatting or anti-virus tools) and device drivers, which act as the interface between a device and the operating system.
Programming tools and language translators. An assembler converts assembly language to machine code. A compiler converts the whole source program to machine code, after which it is not needed to run the program. An interpreter translates and runs one line at a time, so it is needed every time the program runs.
Application software, general purpose (a spreadsheet, a browser) or customised (school management software).
NCERT also notes that a computer can work without application software but not without system software. The operating system manages processes, memory, files and devices, which is why NCERT calls it a resource manager.
Which mistakes cost marks?
Calling RAM permanent or ROM temporary.
Saying the CPU reads the hard disk directly. Data goes through main memory.
Using 1000 instead of 1024 in conversions without being asked.
Mixing up compiler and interpreter. The compiler translates the whole program first; the interpreter goes line by line.
Calling a device driver application software. It is system software.
A practice routine
Draw the memory ladder from cache to secondary storage, labelling speed, size and volatility.
Do five conversions each way: GB to bytes, and bits to KB.
Classify ten programs on your computer as system, translator or application software.
Explain, in three sentences, what happens on the buses when the CPU reads one byte from RAM.
If you want to see how these parts fit together, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, each with its own lesson and its place in the map.
Before your next class, convert 3 GB to bytes using NCERT's table, then check that dividing by 1024 three times brings you back.
