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CSC 230
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Lab 6 Subroutines
Submit lab6.asm at the end of your lab.
I. Subroutines
In lab 4, we learned how to write a simple subroutine, that is, a subroutine without parameter
passing nor a returned value. For example, in C or Java, the prototype of a simple subroutine looks
like this:
void delay()
In today’s lab, we are going to learn how to write subroutines with parameter(s) and returned value.
Example 1: passing two parameters by value using a stack on SRAM (the data memory), the
returned value is saved in register pair sumH:sumL (r19:r18). In C or Java, the equivalent
subroutine looks like this:
int add_num (int n1, int n2)
In assembly language, the parameters n1 and n2 are pushed onto the stack before calling the
subroutine add_num. Download params.asm and add it to your project.
The diagram of the internal memory of the SRAM when “ldd n1, Z+9” is executed:
Address content details notes
0x0000 ~
0x001F General Purpose
Registers
0x0020 ~
0x005F 64 I/O Registers
0x0060 ~
0x01FF
416 extended I/O
Registers
0x0000 ~
0x01FF
……
0x0200 .DSEG
……
0x21F6 <-SP “0x21F6” is stored in SP (Stack Pointer)
0x21F7 n2 (r1) saved register In the subroutine “add_num”, the callee pushes
those registers which will be used onto the stack
to protect them from being changed.
0x21F8 n1 (r0) saved register
0x21F9 ZH (r31) saved register
0x21FA ZL (r30) saved register
0x21FB ret return address The return address (the address of the next
command) is pushed onto the stack automatically
when “call add_num” is executed.
0x21FC ret return address
0x21FD ret return address
0x21FE 0xCC parameter (Z + 8) In the main, the caller pushes the arguments onto
0x21FF 0xEE parameter (Z + 9) the stack.
CSC 230 Fall 2019
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Three bytes are used for the return address since size of the flash memory is 256KB (218 bytes)
(refer to page 7, Table 2-1 of the datasheet).
Build params.asm and run the code, press F11 and observe the contents of the registers: SP,
Registers Z, r16, r1 and r0, r19:r18
Example 2: passing parameter by reference using a stack on SRAM (the data memory), no returned
value.
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.
Read the code in lab6.asm. Download stackFramePractice.docx and write the stack frame for strcpy
using the example on the previous page.
Address content details notes
……..
<- SP Stack Pointer
…… In the subroutine, the callee pushes those
registers onto the stack in order to preserve the
values in the registers.
ret return address The return address (the address of the next
command) is pushed onto the stack automatically
when “call strcpy” is executed.
ret return address
ret return address
…..
In the main, the caller pushes parameters on to
the stack: the memory address of the destination
string.
In the main, the caller pushes parameters on to
the stack: the memory address of the source
string.
0x21FF
CSC 230 Fall 2019
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Design the stack frame for strlength(String str):
Address content details notes
……..
<- SP Stack Pointer
…… In the subroutine, the callee pushes those
registers onto the stack in order to preserve the
values in the registers.
ret return address The return address (the address of the next
command) is pushed onto the stack automatically
when “call strlength” is executed.
ret return address
ret return address
…..
In the main, the caller pushes parameter on to the
0x21FF stack: the memory address of the source string.
Submit lab6.asm at the end of your lab.