Computing - AQA Assembly

Created: October 02, 2020

What is a register?

A temporary storage area in a CPU.

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

Registers.

What does LDR Rd, (memory ref) do?

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

How many arguments does LDR take?

2

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

LDR.

What does LDR R1,300 do?

Load the contents of address 300 into R1.

What does STR Rd, (memory ref) do?

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

How many arguments does STR take?

2

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

STR.

What does STR R3,202 do?

Store what is in R3 at memory address 202.

What does ADD Rd, Rn (operand) do?

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?

3

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

ADD.

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

ADD R1,R1,#3

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

ADD.

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

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

How many arguments does SUB take?

3

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

SUB.

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

SUB R5,R7,#5

What does MOV Rd,(operand) do?

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

How many arguments does MOV take?

2

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

MOV.

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

MOV R0,#7

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

Using compare and branch.

What does CMP Rn,(operand) do?

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

How many arguments does CMP take?

2

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

CMP R10,300

What are the 4 possible results of a comparison?
  • EQ
  • GT
  • NE
  • LT

What does B <label> do?

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

How many arguments does B take?

1

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

B end

What does B<condition> <label> do?

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

How many arguments does B<condition> take?

1

What does BNE mean?

Branch if not equal.

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

BEQ.

How can you write a while loop in assembly?
  • Have a label at the start of the loop
  • Write condition that branches to start if met

What are the instructions for the 4 logical bitwise operators?
  • AND
  • ORR
  • EOR
  • MVN

What arguments do all the logical bitwise instructions take?

Rd, Rn, (operand)

What does EOR R1, R3, #5 do?

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?

Think “exclusive or”.

How can you test if bits are set in assembly?
  • 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?
  • 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?

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?

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

What are the two logical shift operators?
  • LSL
  • LSR

What does LSL do?

A left logical shift (increasing).

What does LSR do?

A right logical shift do??

What arguments do LSL/LSR take?

LSL/LSR Rd,Rn,(operand)

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

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

What does the HALT instruction do?

Stop the execution of the program.

What are the two types of (operand)?
  • #, a value.
  • Rm, the value stored in register m.

What does Rd represent?

“register destination”.

What does Rn represent?

“register number”.



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