CS306 - Lecture 3

CS306

Processing Systems and Structures

J. W. Lockwood,

Lecture 5

About this Lecture

Recall from lecture 4: Logical and arithmetic operations set flags.
Recall from lecture 2: Flags are like a registers:
They retains state information from previous instruction(s)
By reading the flags, we can perform conditional jumping
i.e., transfer execution of the program to elsewhere in the program.
- If condition was true:
  - Jump back in your back or forward in your code to the location that you specify.
    (Instruction Pointer (IP) gets updated)
- if condition was false:
  - Continue execution at the following instruction
    (IP just gets incremented as usual)

Setting the flags
- CMP X,Y : Compare X to Y - Set flags accordingly
  Read this from left to right. How does X compare to Y.
- SUB X,Y : Calculate X = X - Y - Save result and set flags.
  This is the same as CMP X,Y, but it saves the result to X
For the 80x86, the following terminology was chosen to differentiate between jump commands that use the carry flag and the overflow flag.
- Above/Below: Unsigned Compare
- Greater/Less: Signed (+/-) Compare
Easy to remember: Just combine the following letters
- J = Jump
- N = Not (optional)
- A/B G/L = Above/Below Greater/Less
- E = Equal (optional)
More about the comparing signed integers..

Command	Description	Condition
`JA`=`JNBE`	Jump if above Jump if not below or equal	C=0 & Z=0
`JBE=JNA`	Jump if below or equal	C=1 \| Z=1
`JAE`=`JNB`=`JNC`	Jump if above or equal Jump if not below Jump if no carry	C=0
`JB=JNA`=`JC`	Jump if below Jump if carry	C=1
`JE`=`JZ`	Jump if equal Jump if Zero	Z=1
`JNE`=`JNZ`	Jump if not equal Jump if not zero	Z=0
`JS`	Jump Sign (MSB=1) [More info]	S=1
`JNS`	Jump Not Sign (MSB=0)	S=0
`JO`	Jump if overflow set	O=1
`JNO`	Jump if no overflow	O=0
`JG`=`JNLE`	Jump if greater Jump if not less or equal	S=O & Z=0
`JGE`=`JNL`	Jump if greater or equal Jump if not less	S=O
`JL`=`JNGE`	Jump if less Jump if not greater or equal [More info]	S^O
`JLE`=`JNG`	Jump if less or equal Jump if not greater	S^O \| Z=1
`JCXZ`	Jump if register CX=zero	CX=0

CMP AX,BX JA true_label ... False Processing ... JMP done_label ... true_label: True Processing ... done_label: Resume execution here

Due to instruction encodings (covered in lecture 3), 80x86 conditional jumps only store a 1 byte (short) displacement after the OPCODE. This limits conditional jumping to within -128 .. +127 bytes relative to the IP.
JMP: Unconditional JUMP (JMP allows for 16-bit displacement jump)
MASM will generate extra code if you need to make a longer jump
NASM requires that the programmer fix the code. For example:

So, the code above could be fixed to: