Anatomy of a Mobile. Steven R. Bagley

Size: px

Start display at page:

Download "Anatomy of a Mobile. Steven R. Bagley"

Gilbert Summers
7 years ago
Views:

1 Anatomy of a Mobile Steven R. Bagley

2 Today Last week, considered the characteristics of a mobile device Today, look at what s inside a mobile

3 Device Characteristics CPU (~1GHz or less) RAM (128MB 1GB) Flash storage (varies, sometimes external) Display (LCD, resolution < 1024x768) WiFi, 3G, Bluetooth, GPS User Input (Touch, accelerometer, etc)

4 Device Characteristics CPU (~1GHz or less) RAM (128MB 1GB) Flash storage (varies, sometimes external) Display (LCD, resolution < 1024x768) WiFi, 3G, Bluetooth, GPS User Input (Touch, accelerometer, etc) This sounds like a Computer!

5 Mobile vs. Computer In many ways, the technology within a mobile device is conceptually to the technology in a computer But it is the way it is put together that is different

6 Inside a computer Several chips CPU North bridge, South bridge RAM Several cards with extra bits on

7 Connections CPU has a bus that connects it to the other devices Originally, this would be an address bus, and a data bus alongside some control buses RAM chips etc. have similar connections Connect CPU to RAM/ROM Plus some control logic, and you have a PC

8 Connections CPU has a bus that connects it to the other devices Originally, this would be an address bus, and a data bus alongside some control buses RAM chips etc. have similar connections Connect CPU to RAM/ROM Plus some control logic, and you have a PC

9 Connections CPU has a bus that connects it to the other devices Originally, this would be an address bus, and a data bus alongside some control buses RAM chips etc. have similar connections Connect CPU to RAM/ROM Plus some control logic, and you have a PC

10 Connections These days, more advanced buses (e.g. HyperTransport) are used However, the number of pins used for the bus defines chip size Typical PC parts won t fit in a mobile Need a different approach

11 Transistors A CPU is a collection of transistors All digital logic devices are built out of Transistors 4 transistors will build a NAND gate From a NAND gate, you can build any logic circuit Intel 4004 has 2300, 8008 has had around cpu had 68k, Intel Core i7 has 731m to over a billion depending on CPU type ARM Cortex A9 ~26m

12 System on a Chip Only use a small amount of the transistors to form a CPU The rest are used to form the other parts of the system External pins connect directly to hardware Called System on a Chip

13 System on a Chip Built block by block from descriptions of separate parts CPU block GPU block Etc

14 System on a Chip Qualcomm SnapDragon Apple A4/A5 Texas Instruments OMAP All share the same CPU part (ARM Cortex A8) But may have different companion technology Typically Mobile CPU parts

15 Gives you some idea of scale middle is the SIM card tray A4 chips is actually a Package-on-Package device from

16 OMAP 44x0 Typical, SoC chip used in mobile devices Motorola Droid Bionic, Samsung Galaxy Nexus Contains two ARM Cortex-A9 CPU 3D GPU (PowerVR SGX) IVA Accelerator Image Signal Processor

17 Discuss the various parts of the chip

18 Package on Package What about RAM? Could add it to the SoC... But RAM uses a lot of space... Space we want to use for useful stuff Separate package on top of the SoC Package-on-Package (or PoP)

19 Package on Package What about RAM? Could add it to the SoC... But RAM uses a lot of space... Space we want to use for useful stuff Separate package on top of the SoC Package-on-Package (or PoP)

20 Package on Package What about RAM? Could add it to the SoC... But RAM uses a lot of space... Space we want to use for useful stuff Separate package on top of the SoC Package-on-Package (or PoP)

21 from

22 RAM RAM also tends to be shared between the CPU and GPU unlike in a computer Not all the RAM in the phone is available for the computer... Code assuming you don't have much...

23 Mobile CPUs Almost all Mobile Devices use ARM CPUs Originated from Acorn computers in the late-1980s Acorn RISC Machine Spun out from Acorn in early-1990s Now, Advanced RISC machine

24 ARM CPU 32-bit CPU Doesn t always have a CPU Sold as a design, not a physical device Great for SoC usage! Various revisions of instruction set (current tend to be ARM v7, but ARM v6 also about)

25 ARM CPU RISC-design (Reduced Instruction Set Chip) Removes instructions that aren t used that often E.g. Divide instructions If you need to divide, you roll your own in software (or the compiler does)

26 ARM CPU 16 registers (+ shadows) Load/Store architecture Each instruction is 32-bits long Makes decoding easy But means loading a constant into a register can be tricky

27 Load and Store Data can only be moved From a register to Memory (store) From memory to a register (load) Not memory to memory Although a variety of addressing modes are allowed

28 Constants Constants must fit into the 32-bit instruction width Can t therefore be the full 32-bits 8-bit + a 4-bit shift gives a wide range of values Also a Move Negated instruction If that doesn t work, load from a literal pool

29 Memory Addresses Also makes finding the addresses of things in memory tricky Done by adding a constant to the PC Sometimes requires two or three instructions to calculate Assembler and compiler have psuedoinstructions to do it for you Although PC is 8 bytes ahead

30 ARM Speed Running at 1GHz means its going to be slow right? Not necessarily GHz-speed tells us cycles per second Actual speed depends on how many cycles an instruction takes ARM aims for one-cycle per instruction

31 ARM Speed x86 instructions can take many cycles If an ARM instruction takes 1 cycle at 1GHz And an x86 instruction takes 3 cycles at 3GHz Which is faster? Speed is not entirely down to clock speed

32 ARM Conditional Execution Often want to conditionally execute some code Traditional approach is to execute a compare Then Branch if the condition is met Branches are expensive (lots of clock cycles)

33 ARM Conditional Execution ARM has another trick up its sleeve Any instruction can be made conditional Removing need for an expensive branch Faster program, smaller footprint

34 Barrel Shifter ARM has a barrel shifter that can be used on any instruction Shift/Rotate the bits within the binary number Good way of multiplying or dividing by powers of two in some cases

35 ARM and Thumb Every ARM instruction is 32bits long Can take up a lot of memory Memory accesses take time Can slow things down Enter Thumb

36 Thumb 16-bit version of the ARM instruction set Variable length instruction set Took the most popular ARM instructions used and encoded them as 16-bit values Why?

37 Why Thumb? Speed ARM instructions require 32bits to be read every instruction Memory reads take time Not all memory is 32bit wide On smaller RAM width, takes multiple reads to get an instruction

38 Why Thumb? By making the instructions smaller, you gain a speed up. 8-bit memory two reads per instruction 16-bit memory, one read per instruction 32-bit memory, one read gives two instructions iphone SDK generates Thumb by default

39 Floating Point and Thumb Thumb doesn t encode FPU instructions CPU would have to branch to ARM instructions to execute it This takes time FPU heavy code is better compiled to ARM, not Thumb Assuming the device has an FPU!

40 Compilers Most of the time, the compiler will take advantage of this Although you may have to manually switch it compile ARM over Thumb

41 ARM big.little ARM's latest processor contains two cores One optimised for performance (big core) One optimised for energy efficiency (LITTLE core) Two Cores are architecturally consistent System can switch between the two as appropriate for the task in hand...

Chapter 2 Logic Gates and Introduction to Computer Architecture

Chapter 2 Logic Gates and Introduction to Computer Architecture 2.1 Introduction The basic components of an Integrated Circuit (IC) is logic gates which made of transistors, in digital system there are