ECE 2560 L 9 Subroutines Department of Electrical

ECE 2560 L 9: Subroutines Department of Electrical and Computer Engineering The Ohio State University ECE 3561 - Lecture 1 1

Subroutines l Subroutines What are they l How they can simplify a program l An example l l Stacks Passing data on the stack l How subroutine calls use the stack l ECE 3561 - Lecture 1 2

Stacks l The stack – an area of memory for storage of data accessed through the PUSH and POP instructions ECE 3561 - Lecture 1 3

The 430 stack l After initialization – where is the stack? l l After initialization the value in the SP register (R 2) is 0 x 0300 (can see this in Code Composer) Placing data on the stack l l Done using a PUSH(. B) instruction PUSH(. B) src Push the source onto stack l l Action is SP-2 -> SP src->@SP Removing data from the stack l l Done using a POP(. B) instruction POP(. B) dst Pop item from stack to destination l Action is @SP-> dst SP+2 -> SP ECE 3561 - Lecture 1 4

Also in code composer l l l l If you view memory it will show you data variables names at their location To see action of stack will do some push and pop actions while watching memory Will push values of 10 copies of $FFFF on the stack Then 10 pops to get the SP back to no data – pop into R 4 Then push $0000, $0110, $0220, etc up to $0990 on the stack Again 10 pops back into R 4 Also show what occurs when one is. B ECE 3561 - Lecture 1 5

In Demo l After initialization, the SP is set to value 0 x 0300. l Remember that the stack grows down in memory and the SP register points to where the last value pushed on the stack is. So when a value is pushed on the stack it, the SP register is decremented by 2 and then the data is stored in memory. ECE 3561 - Lecture 1 6

Binary Multiplication l l Need to perform integer multiplication of A*B. Will do this by simply adding up the value B, A times in a loop. l mult l l l mlp mov clr add dec jne A, R 5 B, R 6 R 7 R 6, R 7 R 5 mlp ; A is in R 5 ; B is in R 6 ; R 7 to accumulate sum ; add R 6 to R 7 ; A=A-1 ; repeat ; end R 7=B*A As register is 16 bits and treated as 2’s complement, can do up to a 7 -bit x 7 -bit multiply for a 14 bit result. l Number of times through the loop? 128 times max l ECE 3561 - Lecture 1 7

Now turn this into a subroutine Could code up the previous instructions each time you need to do a multiply OR create a routine to do it. The routine needs the data to multiply passed to it. l Where to pass the data? l l l Could do it in registers Could do it in memory Could do it on the stack Set up before call l l move A and B to stack use push instruction ECE 3561 - Lecture 1 8

Why one an not the other? l Why pass the data on the stack? l Pass it in registers l l Pass in variables in memory l l Works fine but program may be using registers for other tasks. Works fine Pass on the stack l Also works fine For a simple routine like this no real advantage of any method. Just shows how to do pass variables on the stack. l Will look at subroutine for all three. l ECE 3561 - Lecture 1 9

Demo of routine – use registers l Pick two register to pass data l l l l Pick R 5, and R 6 for numbers to be multiplied Pick R 7 for the result Code it up – calling routine mov A, R 5 mov B, R 6 call #mult end jmp end ; A is in R 5 ; B is in R 6 l l l l ; the subroutine mult clr mlp add dec jne ret R 7 R 6, R 7 R 5 mlp ; R 7 to accumulate sum ; add R 6 to R 7 ; A=A-1 ; repeat ; end R 7=B*A l l l . data A B RES . word 3 5 0 ECE 3561 - Lecture 1 10

Subroutine call/return l At end of subroutine code need a ret instruction. (see manual) RET l l Note action of instruction @SP PC SP + 2 SP The CALL instruction action – again see manual. l l dst SP-2 PC tmp SP @SP PC (tmp is not a register you can see) ECE 3561 - Lecture 1 11

Demo of routine – use memory l Pick memory locations to pass data l l l l A and B are the data to be multiplied RES is the result Code it up – calling routine mov #3, &A mov #4, &B call #mult end jmp end l l l l ; the subroutine mult clr mlp add dec jne ret &RES &B, RES &A mlp ; RES to accumulate sum ; add B to RES ; A=A-1 ; repeat ; end RES=B*A l l l . data A B RES . word 3 5 0 ECE 3561 - Lecture 1 12

A third way – the stack l First two methods work, BUT l Limitations REGISTER USE is fixed l Subroutine restricted to specific registers l For subroutine to have no side effects, must protect any register it uses. l ECE 3561 - Lecture 1 13

The main and subroutine l In Main program l l l l l ; Setup for call mov A, R 5 mov B, R 6 push R 5 push R 6 call #srmult ; after return have to cleanup SP will be pointing to B – result is A pop stack twice – 2 nd time is result position ECE 3561 - Lecture 1 14

Subroutine l Now l in subroutine srmult l l l l mlp mov clr add dec jne mov ret 2(SP), R 6 ; B to R 6 4(SP), R 5 ; A to R 5 R 7 R 6, R 7 R 5 mlp ; at end R 7 is A*B R 7, 4(SP) ; R 7 for rtn ; return from sr ECE 3561 - Lecture 1 15

But the call can be a problem l l Have to make sure that call works correctly The first time this was done, call was transferring control all over the place or so it seemed. l l l l ; in main mov push mov call pop push call pop mov A, R 5 B, R 6 R 5 R 6 #smult, R 4 R 7 R 5 R 6 #smult R 7 R 7, res ; subroutine smult mov ret 2(SP), R 6 4(SP), R 5 ; this code worked ECE 3561 - Lecture 1 16

Get code working l When having problems get some code working and then build on it In this case will build the subroutine l Demo l ECE 3561 - Lecture 1 17

Multiply routine l The multiply routine is important as there is no multiply instruction l Note that the routine is for positive integer multiplication only. l With the algorithm implemented cannot multiply a negative number l Both A and B and the result have to be between 0 and 215 -1, positive integers ECE 3561 - Lecture 1 18

Clean up the stack After returning from subroutine A ret simply pops the return address from the stack into the PC register l So execution continues after the subroutine call l If data was passed on the stack it needs to be cleaned up to retrieve the result and set the SP to before the call. l l l ECE 3561 - Lecture 1 19

The pop instruction l The pop instruction removes the top item of the stack (the item currently pointed to by the SP register) and moves it to dst. l POP(. B) dst destination l Pop item from stack to Action is @SP-> dst SP+2 -> SP ECE 3561 - Lecture 1 20

Cleanup on Return l Same code – note the pop instructions that remove data and result from the stack l l l ; in main mov push mov call pop mov A, R 5 B, R 6 R 5 R 6 #smult, R 4 #smult R 7 R 7, res ; subroutine smult mov ret 2(SP), R 6 4(SP), R 5 ; this code worked ECE 3561 - Lecture 1 21

Subroutine side effects l Subroutine should have no side effects!, i. e. , change the values of registers or data the main program is using l l l If subroutine should have no side effects then the state of machine (registers, memory, etc. ) should be the same after the RTS as before the call. What does that mean? That means that subroutine needs to preserve any register that it will affect during execution of the subroutine. What are they? (refer to slide 11) l l CALL and RET do not affect SR Action in subroutine will alter sr bits, and here contents of R 5, R 6, and R 7 ECE 3561 - Lecture 1 22

Items to preserve l The l first item to preserve is the SR How to do it? Push it onto the stack. l What else to save? Any register used. ECE 3561 - Lecture 1 23

Saving of the SR (R(2)) l Save the register you will use and SR. l Becomes ------- -> State of SR upon l return is same as it l was before the call. l ECE 3561 - Lecture 1 24

Save other registers l Subroutine used R 5, R 6, R 7 – push on Stack ECE 3561 - Lecture 1 25

What is the code? l Note the change to the offsets of the passed arguments. Now subroutine has no effect on l calling program. l ECE 3561 - Lecture 1 26

Assignment l Code up a positive integer multiply subroutine as shown in this lecture l Description of this is on the webpage and is assignment HW 6 ECE 3561 - Lecture 1 27