Allocating Memory

In Lab 7, you allocated memory for a truth table using the .byte assembly directive.  For this lab, you will need to allocate other blocks of memory. 

The online manual describes several assembly directives for allocating space.  The information is summarized below:

For example, to declare a global variable nVal in your code, insert the following directives and labels, preferably at the end of your code:

.data        ; Not required, but if used, only one is needed
                          ; preceding blocks of memory being allocated

nVal:
.byte     0

This code allocates 1 byte of memory with the initial value of zero.  To access the block of memory, you would need to follow the standard conventions used in Lab 7.

    lis r5, nVal@h
    ori r5, r5, nVal@l

    li r6, 200       ; r6 <- the value 200
    stb r6, 0(r5)    ; Write to nVal, r6 -> nVal

This code segment sets the byte for nVal equal to the value 200.  You can of course read/write the data as necessary.

Allocating a block of memory

For this lab, it will be necessary to allocate a block of data for a buffer.  You may want to use a block of 128, 256 or more bytes of memory for the buffers.  To allocate a block of 256 bytes, use the following code:

QT_Buffer:
.fill    256

To access the memory, you could use the following code:

lis r10, QT_Buffer@h
ori r10, r10, QT_BUffer@l

lbz r15, 5(r10)            ; r15 <- QT_Buffer[5]