Subroutines JSR JSRR - Computer & Information Science

What about User Code?
Service routines provide three main functions
1. Protect system resources from malicious/clumsy
programmers
2. Shield programmers from system-specific details
3. Write frequently-used code just once
Do these benefits apply to application code, too?
CSE 240
JSR
JSR
CSE 240
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
0 1 0 0 1 PCoffset11
R0
R1
R2
R3
R4
R5
R6
R7
Register File
0100000000011001
Just like JMP (but PC is saved in R7)
Why not just use TRAP?
PC 0 1 0 0 0 0 0 0 0 0 0 1 1 0 0 1
BR 512
0 1 0 0 1 0 1 0 0 0 0 0 0 0 0 0
IR
16
0 1 0 0 0 0 1 0 0 0 0 1 1 0 0 1
ADD
SEXT
B
9
16
0000001000000000
A
ALU
16
1 0
16
16
16
9-23
Note: This is PC
of next
instruction
9-25
Subroutines
A subroutine is a program fragment that. . .
• Resides in user space (i.e, not in OS)
• Performs a well-defined task
• Is invoked (called) by a user program
• Returns control to the calling program when finished
Like a service routine, but not part of the OS
• Not concerned with protecting hardware resources
• No special privilege required
Virtues
• Reuse useful code without having to keep typing it in (and
debugging it!)
• Divide task among multiple programmers
• Use vendor-supplied library of useful routines
CSE 240
JSRR
JSRR
Virtues of JSRR?
CSE 240
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
0 1 0 0 0 0 0 BaseR 0 0 0 0 0 0
JSRR R5
0 1 0 0 0 0 0 1 0 1 0 0 0 0 0 0
IR
Note: This is PC of
next instruction
R0
R1
R2
R3
R4
R5
R6
R7
Register File
000001000011000
0100010000011001
16
1 0
16
0 0 0 0 0 1 0 0 0 0 0 1 1 0 0 0
PC 0 1 0 0 0 1 0 0 0 0 0 1 1 0 0 1
this zero means “register mode”
16
16
9-24
9-26
1

Returning From a Subroutine
The RET instruction
• Just a special case of JMP
RET == JMP R7
• Same idea as returning from TRAPs
Note
• If we use JMP to call subroutine, we can’t use RET!
• Why not?
CSE 240
Passing Information To Subroutines
Argument(s)
• Value passed in to a subroutine is called an argument
• This is a value needed by the subroutine to do its job
• Examples
2sComp: R0 is number to be negated
CSE 240
OUT: R0 is character to be printed
PUTS: R0 is address of string to be printed
How?
• In registers (simple, fast, but limited number)
• In memory (many, but awkward, expensive)
• Both
9-27
9-29
Example: Negate the value in R0
2sComp NOT R0, R0 ; flip bits
ADD R0, R0, #1 ; add one
RET ; return to caller
To call from a program
; need to compute R4 = R1 - R3
ADD R0, R3, #0 ; copy R3 to R0
JSR 2sComp ; negate
ADD R4, R1, R0 ; add to R1
...
Note: Caller should save R0 if we’ll need it later!
CSE 240
Getting Values From Subroutines
Return Values
• A value passed out of a subroutine is called a return value
• This is the value that you called the subroutine to compute
• Examples
2sComp: negated value is returned in R0
CSE 240
GETC: character read from the keyboard is returned in R0
How?
• Registers, memory, or both
• Single return value in register most common
9-28
9-30
2

Calling Conventions
Caller/Callee must agree on argument/ret-val location
Approach 1
• Every subroutine does what it likes
• Program needs to look at documentation for each one
Approach 2
• Define a consistent calling convention
LC-3
• First 4 arguments passed in R0, R1, R2, R3
• Subsequent arguments passed in memory (more on this later)
• Single value returned in R5
CSE 240
Saving and Restore Registers
Like service routines, must save and restore registers
• Who saves what is part of the calling convention
Generally use “callee-save” strategy, except for ret vals
• Same as trap service routines
• Save anything that subroutine alters internally that shouldn’t be
visible when the subroutine returns
• Restore incoming arguments to original values (unless
overwritten by return value)
Remember
• You MUST save R7 if you call any other subroutine or trap
• Otherwise, you won’t be able to return!
CSE 240
9-31
9-33
Using Subroutines
Programmer must know
• Address: or at least a label that will be bound to its address
• Function: what it does
CSE 240
NOTE: The programmer does not need to know how the
subroutine works, but what changes are visible in the
machine’s state after the routine has run
• Arguments: what they are and where they are placed
• Return values: what they are and where they are placed
9-32
3