Lab 6 Subroutines

CSC 230 
1
Lab 6 Subroutines
Submit lab6.asm at the end of your lab.
I. Subroutines
In first year programming language courses such as csc 110, csc 111, we wrote many functions. In
assembly language, the term procedure, function, or subroutine can be used interchangeably. A
function call implies a transfer (branch, jump) to an address representing the entry point of the
function, causing execution of the body of the function. When the function is finished, another
transfer occurs to resume execution at the statement following the call. The first transfer is the
function call (for invocation), the second transfer is the return. Together, this constitutes the
processor’s call-return mechanism.
Example: passing two parameters by values. Download “params.asm” (kindly provided by Jason)
The diagram of the internal memory of the SRAM when “lds ZH, Z+9” is executed:
Address content details notes
0x0000 ~
0x01FF
General Purpose
Registers and I/O
Registers
0x0200 .DSEG
……..
 <Z-and SP
r1 saved register Programmer pushes those registers onto the stack
r0 saved register in the subroutine
r31 saved register
r30 saved register
ret return address Assembler pushes the return address onto the stack
automatically when “call do_something” is
executed.
ret return address
ret return address
0xCC parameter (Z + 8) Programmer pushes the values onto the stack in the
0x21FF 0xEE parameter (Z + 9) “main” in the example.
Build params.asm and run the code, observe the contents of the registers SP, Registers Z, r16, r1
and r0.
II. Exercises: download lab6.asm, read void strcpy (src, dest) subroutine. Understand it,
reconstruct the stack frame. Implement unsigned int strlength (str) subroutine using the two
subroutines we have just learnt as examples.
CSC 230 Fall 2016
2
Read the code in lab6.asm. Write on a piece of paper the stack frame for strcpy.
Address content details notes
……..
 < SP
…… Programmer pushes those registers onto the stack
in the subroutine
ret return address Assembler pushes the return address onto the stack
automatically when “call do_something” is
executed.
ret return address
ret return address
…..
Programmer pushes the parameters (could be
values and/or addresses) onto the stack in the
“main” in the example. 0x21FF
Design the stack frame for strlength
Address content details notes
……..
 < SP
…… Programmer pushes those registers onto the stack
in the subroutine
ret return address Assembler pushes the return address onto the stack
automatically when “call do_something” is
executed.
ret return address
ret return address
…..
Programmer pushes the parameters (could be
values and/or addresses) onto the stack in the
“main” in the example. 0x21FF
Submit lab6.asm at the end of your lab.