1 Investigating CPU Instructions
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Investigating CPU Instructions Objectives of this tutorial At the end of this lab you should be able to: Enter CPU instructions using the CPU simulator Describe the effect of compare instruction on CPU status flags Z and N Construct a loop and explain jump instructions that use Z and N flags Use direct addressing to access memory location(s) Use indirect direct addressing to access memory location(s) Construct a subroutine and call it Pass parameter(s) to the subroutine Special instructions Save your work at regular intervals. The CPU simulator is work in progress so it may crash from time to time. If this happens then restart it and load your latest code, hence the importance of the above statement! Ask if you need help with any aspect of this tutorial. The simulator is there to help you understand the theory covered in lectures so make the most of it. 1 Exercises Learning objectives: To enter CPU instructions and describe effect of compare instruction on CPU status flags Z and N. In theory In practice The CPU instruction sets are often grouped together into categories containing instructions with related functions. 1. Enter the following code and run it: This exercise familiarises you with the way the instructions are entered using the simulator. 2. Add the following code and note the states of the status flags Z and N after each compare instruction is executed: The most encountered instructions involve data movements, e.g. the MOV instruction. Another common instruction is the comparison instruction, e.g. CMP. This instruction sets or resets the status flags Z and N as a record of the result of the comparison operation. MOV #1, R01 MOV #2, R02 ADD R01, R02 CMP #3, R02 Z N CMP #1, R02 Z N CMP #4, R02 Z N Explain your observations of the states of the status flags after the compare instructions above: ** Now save the above code! ** Learning objectives: To construct a loop and explain jump instructions that use Z and N flags In theory In practice The computer programs often contain loops where the same sequence of code is repeatedly executed until or while certain condition is met. Loops (or iterative statements) are the most useful features of programming languages. 3. Add the following code and run it (ask your tutor how to enter a label): At the instruction level, loops use conditional jump instructions to jump back to the start of the loop or to jump out of the loop. This exercise is created to demonstrate the use of the jump instruction which often uses the result of a compare instruction. Such jump instructions use the Z and the N status flags to jump or not jump. MOV #0, R01 Label1 ADD #1, R01 CMP #3, R01 JLT $Label1 Summarize what the above code is doing in plain English: 4. Modify the above code such that R01 is incremented by 1 until it reaches the value of 4. Copy the code below: ** Now save the above code! ** 2 Learning objectives: To use direct addressing to access memory location(s) In theory In practice Although instructions moving data to or from registers are the fastest instructions, it is still necessary to move data in or out of the main memory (RAM) which is a much slower process. 5. Add the following code and run: Examples of instructions used to store into or get data out of memory are explored here. The method used here uses the direct addressing method, i.e. the memory address is directly specified in the instruction itself. STB #h41, 16 LDB 16, R03 ADD #1, R03 STB R03, 17 Make a note of what you see in the program’s data area: What is the significance of h in h41 above? Modify the above code to store the next two characters in the alphabet. Write down the modified code below: ** Now save the above code! ** Learning objective: To use indirect addressing to access memory location(s) In theory In practice There are circumstances which make direct addressing unsuitable and inflexible method to use. 6. Add the following code and run: In these cases indirect addressing is a more suitable and flexible method to use. In indirect addressing, the address of the memory location is not directly included in the instruction but is stored in a register which is the used in the instruction. This exercise introduces an indirect method of accessing memory. This is called register indirect addressing. There is also the memory indirect addressing but this is left as an exercise for you. Label2 MOV #16, R03 MOV #h41, R04 Label3 STB R04, @R03 ADD #1, R03 ADD #1, R04 CMP #h4F, R04 JNE $Label3 Make a note of what you see in the program’s data area: Explain the significance of @ in @R03 above: ** Now save the above code! ** 3 Learning objective: To construct a subroutine and call it In theory In practice Another very useful feature of programming languages is subroutines. These contain sequences of instructions which can be executed many times. If a subroutine was not available the same sequence of instructions would have been repeated many times increasing the code size. 7. Add the following code but do NOT run it yet: At instruction level, a subroutine is called by an instruction such as CAL. This effectively is a jump instruction but it also saves the address of the next instruction. This is later used by a subroutine return instruction, e.g. RET to return to the previous sequence of instructions. MSF CAL $Label2 Now convert the code in (6) above into a subroutine by inserting a RET as the last instruction in the subroutine. Make a note of the contents of the PROGRAM STACK after the instruction MSF is executed (see tutor what to do to facilitate this observation): Make a note of the contents of the PROGRAM STACK after the instruction CAL is executed: What is the significance of the additional information? ** Now save the above code! ** Learning objective: To pass parameter(s) to the subroutine In theory In practice Useful as they are the subroutines are not very flexible without the use of the subroutine parameters. These are values passed to the subroutine to be consumed inside the subroutine. 8. Let’s make the above subroutine a little more flexible. Suppose we wish to change the number of characters stored when calling the subroutine. Modify the calling code in (7) as below: The common method of passing parameters to the subroutines is using the program stack. The parameters are pushed on the stack before the call instruction. These are then popped off the stack inside the subroutine and consumed. This exercise is created to demonstrate this mechanism. MSF PSH #h60 CAL $Label2 Now modify the subroutine code in (6) as below and run the above calling code: Label2 MOV #16, R03 MOV #h41, R04 POP R05 Å Label3 STB R04, @R03 ADD #1, R03 ADD #1, R04 CMP R05, R04 Å JNE $Label3 RET Add a second parameter to change the starting address of the data as a challenge and write the code in the box above! ** Now save the above code! ** 4
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