CS2.201 - Computer Systems Organization | CSO Lecture 7 notes

Operand specifiers

Suppose the initial state of memory and registers looks like the following:

Memory          Register

Addr   Val      Reg   Val
-----------     ----------
0x100  0xFF     %eax  0x100
0x104  0xAB     %ecx  0x1
0x108  0x13     %edx  0x3
0x10C  0x11

Now if we were to execute one of the following instructions, (not sequentially, just any one instruction), we would get:

1. movl $0xFFFF %eax Store the value 0xFFFF in %eax

2. movl %eax %edx Copy the value of register %eax into %edx

3. movl 0x108 %edx Copy the value at memory address 0x108 into %edx

4. movl (%eax) %edx Interpret the value of %eax as a memory address, and copy the value at that address to %edx

5. movl 0x8(%eax) %edx Interpret the value of %eax as a memory address, offset it by 0x8, and copy the value at this calculated address to %edx

6. movl (%eax, %ecx) %edx Interpret the value of %eax as a memory address, offset it by the value at %ecx, and copy the value at this calculated address to %edx

7. movl 0x100(%eax, %ecx, 0x1) %edx Interpret the value of %eax as a memory address, offset it by the value at %ecx times the constant 0x1, offset this further by the constant 0x100, and finally copy the value at this calculated address to %edx

Data Movement Instructions

The most basic data movement instructions are the following:

• movb - move byte
• movw - move word
• movl - move double word
• movq - move quad word (64-bit only)

When copying from a smaller field to a larger field, we have to take care of the sign and padding with either 0’s or 1’s appropriately. For this we have the following:

• movsbw - move sign-extended byte to word
• movsbl - move sign-extended byte to double word

• movswl - move sign-extended word to double word

• movzbw - move zero-extended byte to word
• movzbl - move zero-extended byte to double word
• movzwl - move zero-extended word to double word

For unsigned copying, movzbq movzwq exist only, since movl already copies with zeros for padding.