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Processor Fundamentals and Assembly Language: Question 5

Syllabus 4.2

Structured AS 6 marks

A processor's instruction set includes the indirect-addressing instruction LDI <address>, which loads into ACC the contents of the address that is itself stored at <address> (rather than the contents of <address> directly, as LDD <address> would). It also includes OUT, which sends the character whose ASCII value is currently held in ACC to the screen.

Some ASCII character codes: 65 represents the character 'A'.

The contents of main memory before the program runs are:

Address 70 80
Data 80 65

The program stored in memory is:

30  LDI 70
31  OUT
32  END

(a) Explain how the indirect addressing used by LDI 70 finds the value that is loaded into ACC, referring to address 70 and address 80 in this scenario. [2]

(b) Trace the execution of this program, stating the value held in ACC after each instruction executes. [2]

(c) State the character that is output when this program runs, and identify which instruction is responsible for producing this output. [2]

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Worked solution

Part (a): How indirect addressing works here

LDI 70 does not load the contents of address 70 directly into ACC. Instead, address 70 is treated as holding a pointer: its contents (80) are themselves used as a second address. The value actually loaded into ACC is the contents of this second address, address 80, which is 65. So LDI 70 follows the pointer stored at address 70 to reach address 80, and loads the value found there.

[2 marks]: 1 for identifying that the contents of address 70 (80) are used as a second address rather than as the value itself, 1 for correctly identifying that the value loaded is the contents of address 80 (65).

Part (b): Tracing the program

  • LDI 70 → ACC = contents of the address stored at address 70 = contents of address 80 = 65.
  • OUT → ACC is unchanged at 65; the character with ASCII value 65 is sent to the screen.
  • END → ACC is unchanged at 65; the program stops.

[2 marks]: 1 for ACC = 65 after LDI 70, 1 for ACC remaining unchanged at 65 through OUT and END.

Part (c): The output character

Since ACC holds 65 when OUT executes, and 65 represents the character ‘A’, the program outputs ‘A’. This output is produced by the OUT instruction at address 31. LDI 70 only loads the value into ACC, and does not itself produce any output.

[2 marks]: 1 for the character ‘A’, 1 for identifying OUT as the instruction that produces the output.

Final answers

  • (a) LDI 70 uses the contents of address 70 (80) as a second address, then loads the contents of that second address (65) into ACC.
  • (b) ACC = 65 after LDI 70; unchanged at 65 after OUT and END.
  • (c) The character ‘A’ is output, produced by the OUT instruction.