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Chapter 22: Multi Loads & Stores

We will go over a couple of instructions that will shorten code which contain multiple/consecutive store/load instructions. These instructions will also get you "prepped" for the next 2 chapters.

Store Multiple Word:

stmw rD, SIMM (rA) #EA = rA + SIMM; if r0 is rA, then it's literal zero

Consecutive words are stored STARTING at EA. The amount of consecutive words stored depends on what register is used for rD.

For example if r30 is used for rD then...

• r30 is stored at EA
• r31 is stored at EA+4

Another example: If r25 is used for rD then...

• r25 is stored at EA
• r26 is stored at EA+4
• r27 is stored at EA+8
• r28 is stored at EA+12
• r29 is stored at EA+16
• r30 is stored at EA+20
• r31 is stored at EA+24

Usage of this instruction when rD = r31 is pointless as a regular stw will suffice.

Example--

stmw r29, 0x00EC (r5)

If...
r5 = 0x40800484
r29 = 0x00000001
r30 = 0x100AFFF4
r31 = 0x100004E4

Then..
The word 0x00000001 will be stored at 0x40800570
The word 0x100AFFF4 will be stored at 0x40800574
The word 0x100004E4 will be stored at 0x40800578

Here's a pic with the instruction and values, right before the stmw instruction is going to execute

Now here's a pic of once the instruction has executed.

The red arrows show how the 3 words were consecutively stored to Memory starting at 0x00000000.

Load Multiple Word:

lmw rD, SIMM (rA) #EA = rA + SIMM; if r0 is rA, then it's literal zero

This is simply the reverse of stmw. Consecutive words are loaded STARTING at the EA. The amount of consecutive words loaded depends on what register is used for rD.

IMPORTANT: The GPR used for rD *MUST* be a higher GPR than rA, or else a Program Exception (invalid instruction) will occur.

If rD is r30, then...

• Word at EA is loaded into r30
• Word at EA+4 is loaded into r31

If rD is r25, then...

• Word at EA is loaded into r25
• Word at EA+4 is loaded into r26
• Word at EA+8 is loaded into r27
• Word at EA+12 is loaded into r28
• Word at EA+16 is loaded into r29
• Word at EA+20 is loaded into r30
• Word at EA+24 is loaded into r31

Using this instruction when rD = r31 is pointless, as a standard lwz instruction can be used instead.

Example:

lmw r29, 0x00EC (r5)

If r5 = 0x40800094 and...
The word at Address 0x40800180 = 0x408003A0
The word at Address 0x40800184 = 0x10000608
The word at Address 0x40800188 = 0x00000000

Then..
r29 would equal 0x408003A0
r30 would equal 0x10000608
r31 would equal 0x00000000

Here's a picture of right before the lmw instruction is going to execute...

Here's a pic of once it has executed..

We see via the blue arrows how 3 consecutive words starting at 0x40800180 were loaded into r29, r30, and r31.

The EA for *any* stmw or lmw instruction *MUST* be divisible by 4 or else an Alignment exception will occur.

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