Computer Systems and Organization
What to know
Computer systems questions test the path from a high-level instruction to physical signals. Know binary, hexadecimal, and two's complement; be able to convert among them and explain why n bits represent 2^n patterns. Understand Boolean gates (AND, OR, NOT, XOR, NAND), combinational versus sequential circuits, and how an ALU, registers, control unit, clock, and memory form the fetch-decode-execute cycle. Distinguish von Neumann's shared instruction/data memory from Harvard's separate paths. Learn the memory hierarchy-registers, L1/L2/L3 cache, RAM, SSD/disk-and why locality makes caches effective. Know virtual memory, paging, page faults, interrupts, and operating-system roles such as process scheduling, file systems, and protection.
Traps
Questions often blur cache misses with page faults, confuse signed and unsigned ranges, or ask for the speed order of storage. Read whether a value is signed. In 8-bit two's complement the range is -128 to 127, so 127 + 1 overflows to -128 rather than 128. Another trap is assuming more bits always means faster; width affects range and precision, not clock speed.
Worked example
Represent -5 in 8-bit two's complement. Write +5 as 0000 0101, invert to 1111 1010, add 1: 1111 1011. Check: -128 + 64 + 32 + 16 + 8 + 2 + 1 = -5. Rule of thumb: for two's complement, flip bits and add one; the leftmost bit has weight -2^(n-1).