Instruction sets, addressing modes and assembly language

The table shows the contents of memory locations before a program runs. A program is run using the AQA assembly language instruction set.

LDR R0, 100
LDR R1, 101
ADD R2, R0, R1
SUB R2, R2, #5
STR R2, 102
HALT

(a) State the value in R2 after the ADD instruction executes. [1]

(b) State the value in R2 after the SUB instruction and the value stored in location 102 when the program ends. [2]

(c) State the purpose of the HALT instruction. [1]

      MOV R0, #1
      MOV R1, #0
loop: ADD R1, R1, R0
      ADD R0, R0, #1
      CMP R0, #5
      BLT loop
      HALT

(a) State the final values in R0 and R1 when the program halts. [2]

(b) State how many times the instruction ADD R1, R1, R0 is executed. [1]

(c) Describe what the program calculates. [1]

(d) State the change needed so that the program calculates the sum of the integers from 1 to 10. [1]

Memory location 200 contains the binary value 10110111.

LDR R0, 200
AND R1, R0, #15
LSL R2, R1, #2
EOR R3, R2, #255
HALT

(a) Give the denary values in R1, R2 and R3 after the program halts. [3]

(b) State the effect of the instruction LSL R2, R1, #2 in terms of multiplication. [1]

(c) State the effect of EOR R3, R2, #255 on an 8-bit value. [1]

Write an assembly language program, using the AQA instruction set, that:

• reads a number from memory location 300;
• stores 1 in memory location 301 if the number is greater than 50;
• otherwise stores 0 in memory location 301;
• then stops.

[6]