CS 3841 Computer Organization II Recitation LAB8

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CS 3841 Computer Organization IIRecitation LAB8

• - Array,Dynamic Mem. Alloc.,Stack -

LI XU
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Memory segment

• Data segment
• Global constants and variables
• Text segment
• Machine language code
• Stack segment
• Parameters, local variables and return address

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Declare array
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Example - Array

• .data
• array1 .word -2,-1,0,1,2
• array2 .space 5
• .text
• la t0, array1 load address of array1 into t0
• lw t1, 0(t0) lw t2, 4(t0)
• or addi t0, t0, 4
• la t2, array2
• lb t3, 3(t2) t3 array23

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Dynamic Memory Allocation

• Here is how a SPIM program requests a block
  of memory from SPIM's heap
• li a0,xxx a0 contains the number of
  bytes you need.
• This must be a multiple of
  four.
• li v0,9 code 9 allocate memory
• syscall call the service.
• v0 lt-- the address of the
  first byte
• of the dynamically
  allocated block
• You don't know in advance what range of
  addresses you will get back for the allocate
  memory request. (This is similar to a call to
  malloc() in "C".)

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MIPS Stack

• The stack is often used to hold temporary values
  when most registers are already in use.
• By convention, sp(29) always points to the top
  of the stack. Also by convention, the stack grows
  downward (in terms of memory addresses).
• On a computer with a full operating system, the
  stack pointer is initialized by the operating
  system before control is passed to a user
  program. In MIPS architecture, Initially sp
  0x7FFFFFFC
• The data elements in our stacks are 32-bit words.
  In general, stacks can be used for all types of
  data. But in this course, stacks contain only
  32-bit full words.

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MIPS Stack

• LIFO
• Last In First Out
• sp (29)
• TOS (top of stack)
• Initially sp 0x7FFFEFFC
• Upside down Stack of words
• The stack grows toward lower address.

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Stack - Push Operation

• To push an item onto the stack, first subtract 4
  from the stack pointer, then store the item at
  the address in the stack pointer.
• PUSH the item in t0
• subu sp,sp,4
• point to the new place
• sw t0,(sp)
• store t0 at the new top

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Stack - Pop Operation

• To pop the top item from a stack, copy the item
  pointed at by the stack pointer, then add 4 to
  the stack pointer.
• POP the item into t0
• lw t0,(sp)
• copy the top item into t0
• addu sp,sp,4
• point to the new place

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Example Reverse String

• Program Outline
• Input the string into a buffer -------------------
  ----(1)
• Push each character onto the stack
  ---------------(2)
• Pop chars from stack back into the buffer
  --------(3)
• Print the reversed string ----------------------
  -------(4)

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First section Input

• input the string
• li v0,8 service code
• la a0,str address of buffer
• li a1,128 buffer length
• syscall
• li t0,0 push a null
• subu sp,sp,4 onto the stack
• sw t0,(sp) to signal its bottom

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Second section Push

• push each character onto the stack
• pushl
• lbu t0,str(t1) get current char into
• a full word
• beqz t0,stend null byte end of
  string
• subu sp,sp,4 push the full word
• sw t0,(sp) holding the char
• stend

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Third section Pop

• pop chars from stack back into the buffer
• popl
• lw t0,(sp) pop a char off the stack
• addiu sp,sp,4
• beqz t0,done null means empty stack
• done

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Fourth section Output

• print the reversed string
• done
• li v0,4 service code
• la a0,str address of string
• syscall
• li v0,10 exit
• syscall

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Exercise

• Reserve and output a user-supplied string
• t0 stores character pushed or popped.
• t1 Index into string buffer
• v0 a0 a1 can be used in system call

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Assignment 2 (100pts)

• Due day 8th April, Wednesday. By 11 P.M.
• Submission
• Three source code files.
• Submit the above files by email to the TA
  (lxu_at_cs.utsa.edu)