Computing - AQA Assembly

AQA Computer Science 2022

Created: October 02, 2020 | Read markdown | About these notes | View in context | Study these flashcards |

Registers

What is a register?

register-definition

A temporary storage area in a CPU.

Simple computers have an accumulator, more complex computers have…?

accumulator-versus-registers

Registers.

Loading and storing

What does LDR Rd, (memory ref) do?

ldr-instruction-effect

Load the value stored in (memory ref) into register d.

How many arguments does LDR take?

ldr-argument-count

2

I want to put a value at a memory address into a register. What instruction should I use?

ldr-use-case

LDR.

What does LDR R1,300 do?

ldr-example

Load the contents of address 300 into R1.

What does STR Rd, (memory ref) do?

str-instruction-effect

Store the value in register d in the memory location specified by (memory ref)

How many arguments does STR take?

str-argument-count

2

I want to store what’s in a register in a memory address. What instruction should I use?

str-use-case

STR.

What does STR R3,202 do?

str-example

Store what is in R3 at memory address 202.

Addition and subtraction

What does ADD Rd, Rn (operand) do?

add-instruction-effect

Add the value specified by (operand) to the value in register n and store the result in register d.

How many arguments does ADD take?

add-argument-count

3

I want to add a number to a register. What instruction should I use?

add-use-case-number

ADD.

I want to increment R1 by 3. What instruction do I use?

add-increment-example

ADD R1,R1,#3

I want to add what’s in a memory address to a register. What instruction should I use?

add-use-case-memory

ADD.

What does SUB Rd, Rn, (operand) do?

sub-instruction-effect

Subtract the value specified by (operand) from Rn and store it in Rd.

How many arguments does SUB take?

sub-argument-count

3

I want to subtract a number from a register. What instruction should I use?

sub-use-case

SUB.

I want to subtract 5 from R7 and store the result in R5. What instruction should I use?

sub-example

SUB R5,R7,#5

Moving values

What does MOV Rd,(operand) do?

mov-instruction-effect

Move the value specified by (operand) into register Rd.

How many arguments does MOV take?

mov-argument-count

2

I want to put a value into a register. What instruction should I use?

mov-use-case

MOV.

I want to put the number 7 in register R0. What instruction should I use?

mov-example

MOV R0,#7

Comparison and branching

How can you do something similar to if-statements in assembly?

if-statements-in-assembly

Using compare and branch.

What does CMP Rn,(operand) do?

cmp-instruction-effect

Compare the value stored in register Rn with the value specified by (operand).

How many arguments does CMP take?

cmp-argument-count

2

I want to compare the contents of register R10 with what’s at memory address 300. What instruction should I use?

cmp-example

CMP R10,300

What are the 4 possible results of a comparison?

comparison-results
  • EQ
  • GT
  • NE
  • LT

What does B <label> do?

b-instruction-effect

Always branch to the instruction at position <label> in the program.

How many arguments does B take?

b-argument-count

1

I want to always branch to the label end in a program. What instruction should I use?

b-example

B end

What does B<condition> <label> do?

conditional-branch-effect

Branch to <label> if the last comparison met the conditions specified by <condition>.

How many arguments does B<condition> take?

conditional-branch-argument-count

1

What does BNE mean?

bne-meaning

Branch if not equal.

What’s the branch instruction for being equal to zero?

beq-zero-branch

BEQ.

Loops

How can you write a while loop in assembly?

while-loop-in-assembly
  • Have a label at the start of the loop
  • Write condition that branches to start if met

Logical bitwise operators

What are the instructions for the 4 logical bitwise operators?

bitwise-operator-instructions
  • AND
  • ORR
  • EOR
  • MVN

What arguments do all the logical bitwise instructions take?

bitwise-instruction-arguments

Rd, Rn, (operand)

What does EOR R1, R3, #5 do?

eor-example

Perform XOR between all the bits in R3 and the number 5 and store the result in R1.

Why is it EOR instead of XOR?

eor-naming

Think “exclusive or”.

How can you test if bits are set in assembly?

testing-set-bits
  • Do a bitwise AND with a binary number comprised only of the bits you want to test for.
  • If the result is the binary number you specified, then it matches.

How can you set bits in assembly?

setting-bits
  • Do a bitwise OR with a binary number comprised of only the bits you want to set.

How can you test for odd numbers using assembly?

testing-odd-numbers

By doing a bitwise AND with #1 and checking if the result is also #1.

I want to test if R3 is even. What are the instructions to do this?

testing-even-instructions
AND R3,R3,#1
CMP R3,#1
BNE <where to go if even>

Logical shifts

What are the two logical shift operators?

logical-shift-operators
  • LSL
  • LSR

What does LSL do?

lsl-meaning

A left logical shift (increasing).

What does LSR do?

lsr-meaning

A right logical shift.

What arguments do LSL/LSR take?

logical-shift-arguments

LSL/LSR Rd,Rn,(operand)

What does LSL Rd,Rn,(operand) do?

lsl-instruction-effect

Left shift what’s in Rn by (operand) places.

Other instructions and operand types

What does the HALT instruction do?

halt-instruction-effect

Stop the execution of the program.

What are the two types of (operand)?

operand-types
  • #, a value.
  • Rm, the value stored in register m.

What does Rd represent?

rd-meaning

“register destination”.

What does Rn represent?

rn-meaning

“register number”.



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