Summary AQA computer science GCSE revision notes

‭Fundamentals‬‭of‬‭Data‬‭representation‬

-‭ ‬ ‭ omputers‬‭use‬‭binary‬‭to‬‭represent‬‭all‬‭data‬‭and‬‭instructions‬
C
‭-‬ ‭Hexadecimal‬‭is‬‭useful‬‭as‬‭it‬‭is‬‭less‬‭characters/‬‭less‬‭digits.‬‭Easier‬‭for‬‭humans‬‭to‬‭understand‬‭than‬‭binary.‬
‭-‬ ‭Data‬
‭●‬ ‭1‬‭kilobyte‬‭=‬‭1000‬‭bytes‬
‭●‬ ‭1‬‭megabyte‬‭=‬‭1000‬‭kilobytes‬
‭●‬ ‭1‬‭Gigabyte‬‭=‬‭1000‬‭megabytes‬
‭●‬ ‭1‬‭Terabyte‬‭=‬‭1‬‭gigabyte‬
-‭ ‬ ‭Character‬‭codes‬‭are‬‭grouped‬‭and‬‭run‬‭in‬‭sequence‬
‭-‬ ‭ASCII‬‭usually‬‭7‬‭bit‬‭ascii.‬‭Unicode‬‭uses‬‭16‬‭bits‬‭which‬‭makes‬‭it‬‭more‬‭useful‬‭to‬‭represent‬‭other‬‭languages‬‭and‬
‭additional‬‭characters‬‭like‬‭emojis.‬
-‭ ‬ ‭A‬‭pixel‬‭is‬‭a‬‭picture‬‭elements‬‭and‬‭is‬‭a‬‭single‬‭point‬‭in‬‭an‬‭image‬
‭-‬ ‭Sound‬‭is‬‭analogue‬‭and‬‭must‬‭be‬‭converted‬‭to‬‭a‬‭digital‬‭form‬‭for‬‭storage‬‭and‬‭processing.‬
‭-‬ ‭Analogue‬‭signals‬‭are‬‭sampled‬‭to‬‭create‬‭the‬‭digital‬‭version‬‭of‬‭a‬‭sound.‬‭This‬‭is‬‭done‬‭by‬‭measuring‬‭the‬
‭amplitude‬‭at‬‭a‬‭point‬‭in‬‭time.‬
-‭ ‬ ‭Sampling‬‭rate‬‭is‬‭the‬‭number‬‭of‬‭samples‬‭per‬‭second.‬‭Sample‬‭resolution‬‭is‬‭the‬‭number‬‭of‬‭bits‬‭per‬‭sample.‬
‭-‬ ‭File‬‭size‬‭=‬‭sampling‬‭rate*resolution*seconds‬
‭-‬ ‭Adding‬‭binary‬
‭●‬ ‭0+0=0‬
‭●‬ ‭0+1=1‬
‭●‬ ‭1+1=0‬‭carry‬‭1‬
‭-‬ ‭Data‬‭compression‬‭is‬‭useful‬‭as‬‭it‬‭reduces‬‭file‬‭size‬‭which‬‭makes‬‭it‬‭easier‬‭to‬‭transmit.‬
‭●‬ ‭Lossy‬‭compression‬‭is‬‭useful‬‭as‬‭some‬‭data‬‭is‬‭removed.‬‭An‬‭image‬‭can‬‭be‬‭compressed‬‭by‬‭reducing‬‭its‬
‭colour‬‭depth‬‭by‬‭reducing‬‭the‬‭range‬‭of‬‭colours.‬‭For‬‭sound‬‭this‬‭can‬‭be‬‭achieved‬‭by‬‭removing‬‭sounds‬
‭out‬‭of‬‭the‬‭human‬‭range‬‭of‬‭hearing‬‭that‬‭were‬‭picked‬‭up‬‭during‬‭recording.‬
‭●‬ ‭Lossless‬‭compression‬‭is‬‭when‬‭file‬‭size‬‭is‬‭reduced‬‭without‬‭the‬‭loss‬‭of‬‭data.‬‭One‬‭method‬‭is‬‭run‬‭length‬
‭encoding‬‭where‬‭runs‬‭are‬‭stored‬‭as‬‭one‬‭item‬‭of‬‭data‬‭instead‬‭of‬‭many.‬‭For‬‭example‬‭if‬‭there‬‭were‬‭6‬
‭whites‬‭then‬‭it‬‭is‬‭lossless‬‭compression‬‭when‬‭they‬‭are‬‭all‬‭stored‬‭as‬‭one‬‭instead‬‭of‬‭separately.‬
‭-‬ ‭In‬‭Huffman‬‭coding‬‭the‬‭letters‬‭that‬‭appear‬‭the‬‭most‬‭have‬‭the‬‭shortest‬‭bit‬‭patterns.‬‭Huffman‬‭coding‬‭is‬‭a‬‭form‬
‭of‬‭lossless‬‭compression‬‭which‬‭makes‬‭files‬‭smaller‬‭using‬‭the‬‭frequency‬‭of‬‭the‬‭characters‬‭in‬‭a‬‭message.‬

‭Computer‬‭Systems‬
‭-‬ ‭Hardware‬‭is‬‭the‬‭physical‬‭components‬‭of‬‭the‬‭computers‬
‭-‬ ‭Software‬‭is‬‭the‬‭programs‬‭that‬‭run‬‭on‬‭a‬‭computer‬
‭-‬ ‭.‬‭to‬‭represent‬‭the‬‭AND‬‭gate‬
‭-‬ ‭+‬‭to‬‭represent‬‭the‬‭OR‬‭gate‬
⊕
‭-‬ ‭ ‬‭to‬‭represent‬‭the‬‭XOR‬‭gate‬
‭-‬ ‭Overbar‬‭to‬‭represent‬‭the‬‭NOT‬‭gate‬
‭-‬ ‭In‬‭AND‬‭gate‬‭if‬‭there's‬‭a‬‭0‬‭its‬‭0‬‭and‬‭in‬‭OR‬‭if‬‭there's‬‭a‬‭1‬‭its‬‭a‬‭1.‬‭And‬‭in‬‭XOR‬‭if‬‭there's‬‭a‬‭1‬‭its‬‭a‬‭1‬‭unless‬‭its‬‭1‬‭and‬‭1‬‭in‬
‭which‬‭case‬‭it'll‬‭be‬‭0‬
‭-‬ ‭System‬‭software‬‭manages‬‭the‬‭computer‬‭system‬‭resources‬‭and‬‭acts‬‭as‬‭a‬‭platform‬‭to‬‭run‬‭application‬‭software.‬
‭-‬ ‭Application‬‭software‬‭is‬‭a‬‭software‬‭that‬‭performs‬‭end‬‭user‬‭tasks‬
‭-‬ ‭An‬‭operating‬‭system‬‭is‬‭a‬‭collection‬‭of‬‭programs‬‭that‬‭manages‬‭and‬‭controls‬‭the‬‭computer.‬‭E..g‬‭microsoft‬
‭windows.‬‭Android‬‭etc.‬‭It‬‭handles‬‭the‬‭management‬‭of‬‭the…‬
‭●‬ ‭Processors‬‭-‬‭allows‬‭programs‬‭to‬‭run‬‭side‬‭by‬‭side‬
‭●‬ ‭Memory‬‭-‬‭allows‬‭more‬‭than‬‭one‬‭program‬‭to‬‭occur‬‭at‬‭once‬‭and‬‭more‬‭than‬‭one‬‭document‬‭open‬‭at‬‭the‬
‭same‬‭time.‬‭OS‬‭determines‬‭how‬‭much‬‭memory‬‭a‬‭program‬‭requires‬‭and‬‭allocated‬‭enough‬
‭pages(memory‬‭blocks).‬
‭●‬ ‭input/output‬‭devices‬‭-‬‭hardware‬‭devices‬‭connected‬‭to‬‭a‬‭computer.‬‭OS‬‭uses‬‭a‬‭device‬‭driver‬‭which‬
‭contains‬‭instructions‬‭on‬‭how‬‭to‬‭control‬‭a‬‭device.‬
‭●‬ ‭Applications‬‭-‬‭application‬‭programs‬‭can‬‭communicate‬‭through‬‭an‬‭interface‬‭from‬‭the‬‭OS.‬
‭applications‬‭manager‬‭allocates‬‭memory‬‭space‬‭and‬‭determines‬‭the‬‭time‬‭needed‬‭for‬‭an‬‭application‬‭to‬
‭run.‬
‭●‬ ‭Security‬‭-‬‭OS‬‭allows‬‭individual‬‭users‬‭to‬‭be‬‭created/deleted,‬‭manages‬‭access‬‭levels‬‭given‬‭to‬‭each‬‭user,‬
‭keeps‬‭a‬‭log‬‭of‬‭files‬‭a‬‭user‬‭deals‬‭with‬
‭-‬ ‭Utility‬‭programs‬‭help‬‭maintain‬‭the‬‭system.‬
‭●‬ ‭Encryption‬‭software‬‭-‬‭uses‬‭a‬‭complex‬‭algorithm‬‭to‬‭scrampls‬‭content‬‭and‬‭only‬‭authorised‬‭users‬‭can‬
‭unscramble‬‭it‬‭and‬‭understand‬‭it.‬
‭●‬ ‭Defragmentation‬‭software‬‭-‬‭A‬‭file‬‭is‬‭usually‬‭stored‬‭as‬‭segments‬‭that‬‭are‬‭split‬‭us‬‭over‬‭a‬‭disk.‬‭They‬‭are‬
‭stored‬‭where‬‭there‬‭is‬‭space‬‭-‬‭called‬‭fragmentation.‬‭This‬‭causes‬‭a‬‭computer‬‭to‬‭run‬‭slower‬‭as‬‭it‬‭takes‬
‭longer‬‭to‬‭read‬‭from/write‬‭to.‬‭Defragmentation‬‭arranges‬‭the‬‭segments‬‭so‬‭they‬‭run‬‭continuously.‬
‭●‬ ‭Data‬‭compression‬‭software‬‭-‬‭Reduces‬‭the‬‭file‬‭of‬‭a‬‭software‬‭as‬‭smaller‬‭files‬‭are‬‭easier‬‭to‬‭transmit.‬
‭●‬ ‭Backup‬‭software‬‭-‬‭To‬‭prevent‬‭data‬‭loss,‬‭regular‬‭copies‬‭of‬‭the‬‭data‬‭are‬‭made.‬‭Backups‬‭are‬‭usually‬
‭made‬‭to‬‭a‬‭high‬‭capacity‬‭secondary‬‭storage‬‭device.‬‭A‬‭full‬‭backup‬‭makes‬‭a‬‭copy‬‭of‬‭every‬‭file‬‭which‬‭is‬