Subroutines Instructions

Key Concepts

Subroutines are like Java functions They are small, self-contained code segments that perform a simple operation. They can take in data and send results back Main program and subroutines share the same registers LC-3 subroutines do not pass in parameters. The are written to expect the main program to pre-load one or more registers with data before calling. If there is an result to return, it is also placed ina register that the main program uses after returning

Terms

Term	Meaning
ALU	The arithmetic and logic unit (ALU) is a component in the CPU that preforms mathematic and logical functions.
Branching	A technique that causes a program to change the flow of its execution; e.g., if/else and calling a function.
Clock Cycle	A single time segment, controlled by the computer's clock, in which circuits receive input values, process the inputs, and then produce an output.
Condition Code (CC)	Information about the previous instruction's result. Can indicate that the previous instruction resulted in an error/exception, overflow/underflow, or positive/zero/negative value
FDE Cycle	The Fetch-Decode-Execute pattern used by the CPU to execute a single instruction.
Instruction	Single executable line of code in a program. Contains OpCode and Operands.
ISA	Instruction Set Architecture. LIst of all Instruction OpCode and expected Operands.
Memory	Storage used by the program to save and retrieve data. This storage is external to the CPU.
OpCode	Assembly instruction keyword as defined by the ISA.
Operand(s)	Assembly instruction parameters, as defined by the ISA.
Register	Storage used by the CPU to save and retrieve data. These devices directly connected to the CPU's control device and the ALU.
Subroutines	Subset of branching that has added capabilities like passing parameters, throwing exceptions, and returning results.
TRAP Routines	Functions that are built-in to the programming language; e.g., System.out.println() in Java.

Introduction

The ability to write small, single-purpose blocks of code that can be used repeatably is a minimal requirement for modern programming languages. It is unlikely that new programming language would succeed today without methods/subroutines

Advantages of using subroutines include:

• Reuse - can be used many times in a program without needing to copy/paste code in multiple places in main program
• Testing - once a subroutine is tested, it can be skipped when troubleshooting the overall program
• Bottom-Up Design - When creating a program, the developer can create and test each function/capability separately. Then built the main program that calls the functions as needed to solve the program

Subroutine instructions in LC-3 provide a capability similar to Java's function calls

public class example {

	public static int addOne(int number) {
	  return number++;
	}

  public static void main(String[] args) {
    int num = addOne(5); //num will be 6
  }
}

Inside the main function, a call to the addOne subroutine, passing 5 in as a parameter.

In addOne the parameter is incremented and returns back to the main program.

Java handles moving the parameter date to the subroutine, switching to the subroutine code, and returning the results back to the main program

In LC-3, assembly can perform the same behavior, but, like most things in assembly, it is a litter more hands-on.

The main mechanism in calling a subroutine is manipulating the PC. Recall the PC controls the next instruction to execute. Without manipulation, the next line wil always be execute. But, by change it, another instruction can be executed next.

The Program Counter register

The Program Counter (PC) register is used my Simulate to control which like of LC-3 code in a program gets executed next

The PC is initially set to the address in the .ORIG xnnnn when to load the assembled program

.ORIG x3000
  
  ADD R0, R0, #10
  ALoop ADD R0, R0, #-1 ; Decrement R0
        BRnz Done       ; R0 is Negative or Zero, get out of the loop
        BRp ALoop       ; R0 is Positive, Loop again
  Done HALT

.END

The Condition Code (CC) register is set based on ADD R0, R0, #-1 (line 4)

• If the result is negative, CC is set to N
• If the result is zero, CC is set to Z
• If the result is positive, CC is set to P

These are mutually exclusive, so one and only one condition can be true at a give time.

BRnz Done (line 5) is a decision point, where Simulate will check the CC, If the CC is N or Z, then this branch will be taken, meaning execution will jump to the instruction at the Done label (line 7)

The branch occurs at the end of the instruction cycle for BRnz Done. Simulate will change the PC to the address of the Done HALT instruction. When the next instruction cycle starts, the Done HALT instruction will be loaded and executed.

BRP ALoop is another decision point. If the previous BR was not taken then this one might, depending on the CC. If this branch is not taken execution will continue at the next instruction.

If the BRP ALoop branch is taken, the PC will be set to the address where the ALoop label sits (line 4). When the next instruction cycle starts, the ALoop ADD R0, R0, #-1 instruction will be loaded and executed.

If BRP ALoop is not taken the PC will remain at its current value.

Anatomy of a Subroutine

For the most part, an LC-3 subroutine is regular LC-3 code, with a couple main requirements:

1. Subroutines include a label on the first line
   • This label is the name of the sub
   • The assembler will determine the address of the first line of the sub, and replaces offest values in lines that call the sub
2. They start by saving all registers they will modify while executing
   • Subs will need to allocate memory to store the contents of these registers
3. They also restore those registers after completing execution
   • Subs will restore the contents of these registers saves at the beginning
4. Finally, subs end with RET to return back to the calling code
   • Actually, since the PC was incremented just before jumping, the code will return to the line after the jump

JMP

Unconditional Jump

LC-3 ISA Format

JMP

OpCode	unused	BaseR
1100	00000	000

OPCode :

unused : not used

BaseR : Register containing the jump address

Examples:

JMP R3 ; Jump to the address in R3

Explanation

The BaseR register is pre-loaded with a 16-bit address of the location to jump to. When the JMP instruction is executed, the PC is loaded with the value in the BaseR. The next instruction cycle will load the instruction at the address contained in the PC.

Unconditional for JMP indicates the instruction does not expect to return back to section of code that performed the JMP. We will see with JSR and JSRR that they provide a means to return to the location where the jump occurred.

Examples

.ORIG x3000
  LD R0, MyJumpPoint
  JMP R0
  HALT

  .FILL MyJumpPoint x0400
.END

R0 will be loaded with the value x0400. This address was loaded because there is some code the user wants to execute at that memory location

.ORIG x3000
  LD R6, MyJumpPoint1
  JMP R0
  HALT
  
  .FILL MyJumpPoint1 xFFE0
.END

R6 will be loaded with the value xFFE0. This address was loaded because there is some code the user wants to execute at that memory location

Gotchas

R7 register is not to be used in any subroutine calls. It is used by Simulate to store the address jumped from so execution switch back to the user code. This is effectively like use as the java return instruction. In fact, the next LC-3 instruction is RET.

RET

LC-3 ISA Format

RET

OpCode	unused	BaseR
1100	00000	111

OPCode :

unused : not used

BaseR : Register containing the return address

Examples:

RET ; Return back from a subroutine call

Explanation

The BaseR register is pre-loaded with a 16-bit address of the location to return to. When the RET instruction is executed, the PC is loaded with the value in the BaseR. The next instruction cycle will load the instruction at the address contained in the PC.

BaseR is hard-coded to 111. This is used as a register address, so it always uses R7 as the BaseR.

R7 is used by Simulate for subroutine instructions. A such, it should not be modified when using the instructions.

Did You Notice...

JMP and RET have the same OpCode?

In fact, they are the same instruction, with one (1) change...BaseR is hard-coded to R7 for RET

When source code with a JMP R2 instruction, the assembler will generate the 16-bit instruction, setting the BaseR to 010 (for R2)

When source code with a RET instruction, the assembler will generate the 16-bit instruction, setting the BaseR to 111 (referencing R7, even though it is not present in the source instruction)

So, you can replace all RET source instruction with JMP R7 and the assembled object code would be identical. RET is an instruction in LC-3 solely to make the source code easier to understand by humans.

Examples

.ORIG x0400
  ADD R3, R2, R1 ; Combine values in R1 and R2
  RET            ; return back to the main program
.END

Gotchas

JSR

Jump Subroutine

LC-3 ISA Format

JSR

OpCode	Mode	PCOffset11
0100	1	00000000000

OPCode :

Mode : 0 for PCOffset Mode, 1 for Relative Mode

PCOffset : Offset from current PC to jump

Examples:

JSR MySub ; Starts executing code at the memory location of MySub

Explanation

JSR will always jump to the location. CC register is not used to decide to jump or not.

Uses an 11-bit PC Offset to calculate a memory address. It can jump -1024 to +1023 memory location before the current PC.

At the end of the JSR's Fetch-Decode-Execute cycle, R7 is set to the current PC. This is the address used by the RET instruction to return back to the original program.

After the JSR instruction executes, the instructions at the PCOffset address begin executing.

Differences between JSR and BR

• CC Register
  • BR checked the CC register to decide to branch or not
  • JSR always jumps
• PC Offset
  • BR used a 9-bit PC Offset. It can jump -256 to +255 memory location before the current PC
  • JSR used a 11-bit PC Offset. It can jump -1024 to +1023 memory location before the current PC
• Returning when done
  • BR does not have a built-in way to return back to where the code branched
  • JSR sets R7 to the current PC before jumping. Using the RET instruction at the end of a subroutine will return to the instruction after the original jump

Examples

.ORIG x3000

  AND R1, R1, #12 ; Load 12 into R1
  JSR TwosComp    ; Apply 2's complement conversion

  HALT

; 2's complement R1 and store in R0
TwosComp NOT R0, R1     ; Flip the bit
         ADD R0, R0, #1 ; Add 1
         RET            ; return back to the main program
.END

Lines 9 and 10 apply the 2's complement algorithm to the value in R1, storing the result in R0

Line 11 is an unconditional jump to the address in R7. See RET Instruction

Line 3 loads a value into R1. R1 is a parameter that the subroutine expect to have been loaded

Line 4 calls the subroutine. After the subroutine returns back to the main program, R0 will contain the 2's complement of the value in R1

Gotchas

R7 cannot be used in the main program or the subroutine. It is used by Simulate to remember the address to return to when the subroutine finished (by calling the RET instruction)

JSRR

Jump Subroutine Relative

LC-3 ISA Format

JSRR

OpCode	Mode	unused1	BaseR	unused2
0100	0	00	000	000000

OPCode :

Mode : 0 for PCOffset Mode, 1 for Relative Mode

BaseR : Register containing 16-bit address to jump to

Examples:

JSRR R4 ; Starts executing code at the memory in R4

Explanation

JSRR works like JSR, but uses a 16-bit register value to jump. JSRR can jump to any memory location in the Simulate memory space.

Note that the OpCode is the same for JSR and JSRR, only the Mode bit is different.

JSRR also sets R7 for the RET instruction and has the same differences to BR as JSR.

Examples

Call built-in subroutine

.ORIG x3000

  AND R1, R1, #12 ; Load 12 into R1
  LEA R4, TwosComp ; Get address of TwosComp subroutine
  JSRR R4    ; Apply 2's complement conversion

  HALT

; 2's complement R1 and store in R0
TwosComp NOT R0, R1     ; Flip the bit
         ADD R0, R0, #1 ; Add 1
         RET            ; return back to the main program
.END

Lines 10-12 are the subroutine to be called

Line 4 loaded the address of the first line of the subroutine into R4

Line 5 uses the value in R4 to jump to the subroutine

Call subroutine loaded separately

.ORIG x3000

  AND R1, R1, #12 ; Load 12 into R1
  LD R4, TwosCompAddr ; Get address of TwosComp subroutine
  JSRR R4    ; Apply 2's complement conversion

  HALT

; 2's complement R1 and store in R0
TwosCompAddr .FILL x5000
.END


;Subroutine assembled and loaded seperately
.ORIG x5000
  ; Executes 2's complement conversion on value in R1
  ; Result is stored in R0
  NOT R0, R1     ; Flip the bit
  ADD R0, R0, #1 ; Add 1
  RET            ; return back to the main program
.END

Lines 14-21 are a separate assembly file assembled and loaded into memory location x5000

Line 10 contains the address of the subroutine

Line 4 loaded the address of the subroutine into R4

Line 5 uses the value in R4 to jump to the subroutine

Gotchas

Save and Restore Registers

We have already stored a register into a memory location as part of assignments where the result of some calculation was to be saved in memory

There is another reason to store registers in memory...to save the state of your code when jumping to a TRAP Routine or a sub function. In both cases, R0-R7 are the only registers for the entire LC-3 Simulate environment. It another function uses a register your code was using, your data will be overwritten. Saving and restoring registers is essential to prevent data loss

Quick Question: Choose One The LC-3 instruction for storing a register R5 to a memory location labeled 'Result' is...
A. STI R5, #12	B. ST Result, R5
C. ST R5, Result	D. R5 .BLKW 1

Does the Main Program or Subroutine save registers The caller code (your code) or the callee code (TRAP or Sub) can both be responsible for preventing register data loss

It would be a waste of clock-cycles and memory for both caller and callee to perform this work The caller code could, but it would have to save all 8 registers because it does not know which registers the callee code will be using

It is more practice for the callee code to save only the registers it knows it will be using, then restore only those registers before returning to the caller code

::: considerit Can the Caller accurately save all 8 registers? The Caller would need to save all 8 register (since it does not know what the TRAP or Sub is going to modify)

However, during the FDE Cycle of the JSR line, R7 will be changed (to current PC). So, the R7 saved before the JSR is executed will be different than R7 during the FDE cycle for the JSR line

If the TRAP/Sub make its own call to another TRAP or Sub, the state of R7 would further change, likely leading to a loss of information needed to correctly return to the main program :::

Finally, the caller code is kept smaller by not having save/restore all 8 registers

True/False It is the Callee's responsibility to save registers that it changes
True	False

R7 - The Way Back Home

During the instruction cycle for TRAP, JSR, and JSRR, R7 is set to the current PC (The next instruction in our caller code) during the FDE cycle

When RET is called by the callee code, the PC will be loaded with the address in R7, which will return the PC back to the caller's next instruction

The callee code is expected to save/restore R7 always. Should the TRAP/User Function call another TRAP/User Function, the way back to the original caller code will be lost

True/False R7 is not important, thus does not need to be saved
True	False

Callee storing registers (Example)

The following Subroutine, mySub will use R1 and R2 internally. As the Callee, it is responsible for saving R1 and R2 before using those registers. Once complete, it will restore both registers, then return to tha Caller

.ORIG x5000

mySub   ST R1, SaveR1 ;save R1
        ST R2, SaveR2 ;save R2
        ST R7, SaveR7 ;save R7

        ;code that changes R1 & R2
        AND R1, R1, #17
        ADD R2, R1, #-12

        LD R1, SaveR1 ;restore R1
        LD R2, SaveR2 ;restore R2
        LD R7, SaveR7 ;restore R7
        RET           ;Return back to caller with R1 & R2 restored

;Allocate 1 memory slots for storing registers
SaveR1  .BLKW 1
SaveR2  .BLKW 1
SaveR5  .BLKW 1

Java vs LC-3 Subroutines

Java example of a call to a user-defined method (subroutine) In this example, main() defines a variable x, then passes a copy of it to the method addOne(). addOne() does something to a copied variable, then passed the result back to the main program

public static void main(String[] args){
    int x = 2;
    int y = addOne(x);
}

public int addOne(int value){
    return value + 1;
}

In LC-3, the code sets R0. It then jumps to addOne label (subroutine name). *addOne code adds 1 to the value in R0, and stores it in R1. Control is returned to the main code for further execution

.ORIG x3000

main    ADD R0, R0, #2  ; Set 2 in R0
        JSR addOne      ; Call subroutine 'addOne'

;Adds 1 to value in R0 and stores in R1
;   Caller sets R0 with a value before calling
;   Sub will add 1 to R0 and store in R1
addOne  AND R1, R1, #0 ; Clear R1 in case there was junk in it
        ADD R1, R0, #1 ; R1 = R0 + 1
        RET

.END
HALT

Main difference between Java and LC-3 Subroutines

Variable Name

Java manages data/values for the code. Variable are created with user-defined names, then the remaining code references values using those names

LC-3's primary variable stores are Registers (R0-R7). It is left to the programmer to remember what each register contains and why

Pass by Value

Java makes a copy of primitive data types (like int) when passing to a method

LC-3 only has 1 set of registers. Any change to a register in a subroutine remains when it returns

Return Value

Java methods return allows the calling code to capture the result in a separate variable name

LC-3, only having 1 set of registers, forces the main and subroutine to pre-agree on which register contains the parameter and which register will contain the return value

True/False A Sub can be designed to get data from caller using memory locations, rather than Registers
True	False

Conclusion

Subroutines make reusable modules that can be called from a main program as needed

R7 is used in subroutines to hold the address to return to once the sub is complete. R7 is set during the FDE cycle of TRAP and Jump instructions

A subroutine must save all registers it uses. Then, restore them before returning

A subroutine ends with the RET instruction, returning execution to the memory location sorted in R7. Hopefully, this is the location of the next line of code in the main (caller) code

Because LC-3 lacks variable names, caller and callee code must agree on which register(s) are used to pass data in and receive data back out of the sub