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MMX In-depth Finally, we get to talk about the MMX in_depth! Before we go any further some points are worth mentioning. First of all we have to have a good understanding about the characteristics of today's computer applications, all the relatively computation-intensive applications. Things like video, audio, graphics and miltimedia applications. There are serveral things all of them have in common,
As we talk more about the MMX set instructions and syntax, the following topics are unescapable.
DATA TYPES Unlike the most of the data types, bytes, words, double words, and quadwords, the major data type of the MMX instruction set is the packed, fixed-point integer. The fixed-point integer is fromed of multiple integer words (bytes, words, double, etc) that are grouped into one single 64-bit quantity. To fully take advantages of the MMX's date types, the size of each fixed-point integer has to be a fixed 64-bit quantuty. This 64-bit quantity is then moved into one of the eight 64-bit MMX registers (we'll talk about those registers later). The decimal point of the fixed-point values is implicit and is left for all the programmers to control for the maximum amount of the programming flexibility. The supported data types are signed and unsigned fixed-point intergers, bytes, words, doublewords and quadwords. The four data types are:
Why is the fixed-point integer run faster than the general 8-bit code? Let's take a example of scaler code and MMX code processing one same image file. Hopofully you will see the difference. The odinary scaler operation can access one data element at a time, that is 8-bit (one pixel) at a time. But the MMX code process pixels at a much faster rate. MMX operation can access 64-bit data at a time that's 8 data element at a time (8 times faster)!!!
REGISTERS The MMX's data types can be stored in any of the eight new MMX registers. And the MMX instructions can access them directly, by using the names from MM0 to MM7.
INSTRUCTIONS MMX has this syntax: instruction [dest, source] The most MMX instructions have two operands: source and destination. MMX instructions typically use use both operand as input and write the result in the destination. The MMX instructions cover several functional units including:
Arithmetic and logical instructions are designed to support the different packed integer data types. These instructions have a different opcode for each data type supported. As a result, the new MMX technology instructions are implemented with 57 opcodes. (more RISC stuff!) MMX use general-purpose, basic instructions that are fast and easily assigned to the parallel pipelines in Intel processors. By using this general-purpose approach, MMX technology provides performance that will scale well across current and future generations of Intel processors. Most instructions have a suffix that indicates operation and data types.
For example, paddusw MM4, mem1 is an MMX instrction. Most of MMX instructions operate on data using wraparound arithmetic. Some MMX instructions oerate on data using saturation arithmetic.
(A table copied from Intel.com) Instruction Recape EMMS : The EMMS instructions empties the MMX states by setting the float point tag to empty (all 1's). Always use EMMS at the end of the runtines!!! Add and Substract instructions:Add and substructact instuctions operate both on signed and unsigned packed data types. (paddb/w/d, psubb/w/d, paddsp/w, psubsb/w, paddusb/w, psubusb/w) Shift instructions: Shift instructions shifts the data elements in the destination operand (psllw/d/q, psraw/d, psrlw/d/q ) Logic instuctions:The logic instructions operate bitwiseon 64 bits. (pand, por, pandn, pxor, ) Multiply instructions:Multiply instructions multiply signed 16 bit word, and product 32 bit doubleword products (pmaddwd, pmulhw, pmullw) Compare instructions:Compare instrctions compares the data in the soure operand and destination operand, and generating masks in the destination operand. (pcmeqb/w/d, pcmpgtb/w/d) Packed and Unpacked instrcutoins: (VERY USEFUL!!) Packed instructions are used to pack bigger data types into smaller ones.Unpacked instrcutions are used the other way around. (packsswb/wb, puncpkhbw/wb/dq, packuswb, puncpklbw/wd/dq) Data transfer instructions:.(VERY USEFUL!!) The data transfer instructions are used to move data to and from MMX registers. Integer registers, and Mem. (movd, movp)
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