Multimedia Support

1
Multimedia Support
Carnegie Mellon University Computer Science
Department Computer Architecture F01
v

• Jernej Barbic
• Overview of multimedia and processors
• Luis von Ahn
• An Example AltiVec.

2
Multimedia means
Creating, displaying, playing, manipulating,
compressing, decompressing

• Still images,
• Audio,
• Video.

Each of these require different type of CPU
operations.
3
Typical multimedia formats

• JPEG
• MP3
• MPEG
• MPEG-4 just released
• Decompression significantly slower than
  compression
• DVD

4
Multimedia operations are a bottleneck
In this presentation, we focus on this

• CPUs cannot compute fast enough
• Memory/disk latencies too large
• Network bandwidth too low
• Playing 352 x 242 x 24 bit MPEG video requires a
  T1 line (1.5 Mbps)

5
The majority of CPU multimedia operations are

• Matrix multiplications, inner products of vectors
• FFT, DCT (discrete cosine transformation)
• JPEG, MPEG, MP3
• Multiply-accumulate instructions
• Calculating the sum of several products (e.g. in
  an inner product)
• Overflow problems
• Product of two 8-bit numbers is a 16-bit number
• Some multimedia architectures incorporate special
  hardware to deal with overflows (saturated
  arithmetics)
• Single precision floating point operations
• Single precision 32-bit
• geometry 3D processing

6
The majority of CPU multimedia operations are
(contd.)

• Calculating the sum of absolute differences
• MPEG compression detecting motion
• Implemented in hardware only in Alphas MVI

detecting motion
7
Multimedia features of modern processors

• PS (paired single) floating point (fp) format
• Problem typical fp registers are 64-bit, but a
  lot of data is 32-bit
• Solution
• pack two 32-bit single precision fp numbers into
  a 64-bit register
• provide hardware to operate on both 32-bit
  components in parallel
• Many geometric operations require only single fp
  precision
• Useful also for non-multimedia tasks (scientific
  computing)
• Using PS increases performance by a factor of two

8
Multimedia features of modern processors (contd.)

• SIMD (single instruction, multiple data)
• SIMD is the integer version of PS
• Divide 64-bit integer registers into eight bytes
  (8 bit) or four half-words (16 bit)
• Operate on this data in parallel
• Parallel arithmetic instructions
• Instructions to store, load, rearrange individual
  bytes

9
Multimedia architectures

• Alpha MVI (motion-video instructions)
  multimedia extension
• Alphas are fast and dont need extensive
  multimedia additions
• Fast video (MPEG) compression enabled in MVI
• Limited SIMD support, little parallel arithmetic
  instructions,
• but already fast without them.
• Special motion detecting instruction
• PERR Ra, Rb, Rc
• Calculates

10
Multimedia architectures (contd.)

• MIPS V
• Fully support SIMD and PS
• Instructions add little to the complexity of
  circuitry
• MIPS MDMX
• More extensive than MIPS V
• Supports 32 media registers
• Media registers mapped to 64-bit floating point
  registers
• SIMD formats oct-byte (8x 8-bit), quad-half (4x
  16-bit)
• Saturated arithmetics avoids underflows and
  overflows
• Wide 192-bit accumulator to support extensive
  multiply-accumulate instructions

11
Multimedia architectures (contd.)

• HP
• Intel
• MMX
• Sun
• VIS (visual instruction set)
• AltiVec

12
Multimedia for the PowerPC AltiVec
Branch Unit
Integer Unit
Floating-Point Unit
Vector Unit
Memory
13
Multimedia for the PowerPC AltiVec

• 32 vector registers
• 162 new instructions
• 128-bit vectors

16 values
8 values
4 values
14
AltiVec Vector Permute
04
08
00
1F
15
09
0A
05
1F
02
03
07
0D
0B
0E
01
VC
VB
1
0
VA
0
1
2
3
4
5
6
7
8
9
A
B
C
D
E
F
VT
vperm VT, VA, VB, VC
15
AltiVec Other Instructions

• Vector Inner Product
• Vector Merge
• Vector Comparisons
• Multiply Accumulate

16
Does This Really Help?
Time
(Times are normalized)