Lecture04 PDF - Embedded Systems Memory

Homework • Reading – PAL pp 119-125, 161-172 – http://www.barrgroup.com/EmbeddedSystems/How-To/Memory-Types-RAM-ROMFlash 1 Memory Architectures • Von Neumann Architecture – Code and data are accessed in one address space – The I/O ports may be in: • The same address space (Memory Mapped I/O) • A...

Homework • Reading – PAL pp 119-125, 161-172 – http://www.barrgroup.com/EmbeddedSystems/How-To/Memory-Types-RAM-ROMFlash 1 Memory Architectures • Von Neumann Architecture – Code and data are accessed in one address space – The I/O ports may be in: • The same address space (Memory Mapped I/O) • A separate I/O address space (Intel) • Harvard Architecture – Code is accessed in one address space – Data and I/O ports are in another address space 2 Memory Architectures • Comparison of Vendor Implementations Program Code Intel 386 Atmega Data I/O Ports Address space Address space Motorola 68xxx Von Neumann Architecture Address space Address space Harvard Architecture 3 Intel Memory Architecture • We’ll discuss various aspects of memory use: – – – – – – Storage of bytes Storage of words and long words Storage of strings The stack and the stack pointer RAM and ROM Addressing - real mode and protected mode 4 Storage of Bytes • Memory is byte addressable (8 bits/byte) • A memory address “points” to the location of a specific byte in memory • As we know from C data types, a “pointer” can be “cast” to point to a type or structure in memory that is larger than one byte • In that case, the memory address “points” to the lowest address of the storage area used 5 Storage of Words and Long Words • Word Sizes – A Word usually means 16 bits or 2 bytes – A Long Word usually means 32 bits or 4 bytes • Many processors (Motorola 68000/PPC and Sun) store words and long words in memory in “Big Endian” fashion • Intel and Atmega Processors store words and long words in memory in “Little Endian” fashion 6 “Big Endian” • Mapping a long word from register to memory: %eax 0x12 0x34 0x56 0x78 *** 0x12 0x34 0x56 0x78 0x1000e0 0x1000e1 0x1000e2 Memory Address *** 0x1000e3 7 “Big Endian” Appearance • Hex value as seen in register: %eax: 12 34 56 78 • Hex value(s) as seen in memory bytes (md): 0x1000e0: 12 34 56 78 • Irrelevant to the end user – self consistent! • Has a logical appearance to a programmer using a debugger! 8 “Little Endian” • Mapping a long word from register to memory: %eax 0x12 0x34 0x56 0x78 *** 0x78 0x56 0x34 0x12 0x1000e0 0x1000e1 0x1000e2 Memory Address *** 0x1000e3 9 “Little Endian” Appearance • Hex value as seen in register: %eax: 12 34 56 78 • Hex value(s) as seen in memory bytes (md): 0x1000e0: 78 56 34 12 • Irrelevant to the end user – self consistent! • A bit confusing to programmers using a debugger! 10 Debugging with “Little Endian” • Mostly learn to live with it! • To help with displays, the “real” Tutor monitor has an "mdd" command (memory-display-doubleword) • This command reorders the bytes and displays four bytes at a time as a double word value: Tutor> mdd 1000e0 1000e0 12345678 ... 11 Storage of Strings • How are strings stored? – Look at string “help” stored at location 0x200000 – Since memory is “byte addressable” each character (i.e., each byte) has its own address – The address of the entire string is the address of the first byte of the string (the lowest address) – The null terminator shows the end ‘h’ 0x200000 ‘e’ 0x200001 ‘l’ ‘p’ ‘\0’ 0x200002 0x200003 0x200004 Memory Address *** 12 The Stack • One register is called the “extended stack pointer (%esp)” and is used to implement a stack • There are various times when data is pushed on the stack and/or popped off the stack that we will study later – Function calls and returns – Interrupts and returns – System calls and returns 13 The Stack • For now, we will just understand how the esp register is used to push and pop data on and off the stack • Why is a specific register used? Can’t the software use any register for a stack pointer? • Not for system stack! Both hardware and software push and pop data on and off the system stack • Hardware is designed to use a specific register (%esp) so software must be consistent and use same register 14 The Stack - Initialization • Software initializes system stack pointer to point just after the end of available area in memory (i.e., not used for another purpose than stack) • Contents of the stack area are not initialized movl $0x01000000, %esp high memory address 0x01000000 ?? 0x00FFFFFF ?? 0x00FFFFFE ?? 0x00FFFFFD ?? 0x00FFFFFC ?? 0x00FFFFFB ?? 0x00FFFFFA ?? 0x00FFFFF9 ?? 0x00FFFFF8 ?? %esp low memory address 15 The Stack – Push Operation • When data is pushed onto the stack 1. The %esp register is first decremented 2. The value being pushed is stored to memory at the address value (“pointer”) stored in the %esp register movl $0x12345678, %eax pushl %eax high memory address 0x01000000 ?? 0x00FFFFFF 0x12 0x00FFFFFE 0x34 0x00FFFFFD 0x56 0x00FFFFFC 0x78 0x00FFFFFB ?? 0x00FFFFFA ?? 0x00FFFFF9 ?? 0x00FFFFF8 ?? %esp before %esp after low memory address 16 The Stack – Pop Operation popl %eax (%eax contains 0x12345678 again) • When data is popped off the stack 1. The value of the data is read from memory at the address value (“pointer”) stored in the %esp register 2. The %esp register is incremented high memory address 0x01000000 ?? 0x00FFFFFF 0x12 0x00FFFFFE 0x34 0x00FFFFFD 0x56 0x00FFFFFC 0x78 0x00FFFFFB ?? 0x00FFFFFA ?? 0x00FFFFF9 ?? 0x00FFFFF8 ?? %esp after %esp before low memory address 17 The Stack – Manually using esp • You can manually place data on the stack by utilizing the esp register as a memory pointer • (%esp) is indirect addressing. It means the memory location points to by the esp register subl $0x04, %esp movl $0x89abcdef, (%esp) high memory address 0x01000000 ?? 0x00FFFFFF 0x89 0x00FFFFFE 0xab 0x00FFFFFD 0xcd 0x00FFFFFC 0xef 0x00FFFFFB ?? 0x00FFFFFA ?? 0x00FFFFF9 ?? 0x00FFFFF8 ?? %esp before %esp after low memory address18 Memory Types • A good reference article: http://www.barrgroup.com/Embedded-Systems/HowTo/Memory-Types-RAM-ROM-Flash • Random Access Memory (RAM) – Contents are “volatile” (lost when power is off) – Can be loaded with code from a non-volatile media/source such as a disk or a network server – Can be used for reading and writing data via normal reads and writes to the memory address – Can be overwritten unintentionally, e.g. using a “bad” pointer when storing data in memory 19 – Faster access Memory Types (cont’d) • 2 types of RAM (depending on the lifetime of data stored): – Dynamic RAM (DRAM) • If power is turn off, its contents will be lost forever. • Even if power is applied, memory is only retained for 4 milliseconds, and it is gone. • Need to be refreshed periodically to keep memory. – Static RAM (SRAM) • If power is turn off, its contents will be lost forever. • Memory is retained as long as the power is on. 20 Memory Types (cont’d) • Read Only Memory (ROM) – – – – Contents are “non-volatile” (saved with power off) Contains code needed to “boot” the processor Can be used for reading (but not writing) data Erasable and Programmable ROM(EPROM): • Low Volume/High Cost (for Development and Testing) • Use ultraviolet light to erase memory – Programmable ROM (PROM): • High Volume/Low Cost (for Production) 21 Software in Embedded System Memory (Low volume / High Cost) Inventory of Blank Memory Chips Dev’t Host File Prototype Board(s) Prom Programmer Manual Process To “Burn” PROMs 22 Software in Embedded System Memory (High Volume / Low Cost) Order Production Quantities Dev’t Host File Memory Chip Vendor Transfer file to Memory Chip Vendor Inventory of Programmed Chips *** Production Boards 23 Hybrid Between RAM and ROM • Electrically Erasable and Programmable ROM(EEPROM) – Erase byte by byte electrically – Once written, the data is stored forever – Higher cost and write cycle is much longer • Flash memory – High density, low cost, non-volatile and fast to read – Electrically erasable a sector (256 to 16 Kbyte) at a time 24 BIOS Memory • For some computer systems (e.g. PC’s) and embedded devices, the BIOS is stored in nonvolatile memory e.g. flash memory. • More details at: http://www.howtogeek.com/180798/where-is-the-bios-stored/ • These parts can be reloaded using a special procedure, e.g. BIOS update for a PC • Risk: If anything goes wrong, the device may become unusable and the part need replacing 25 Difference between Memory Address and Memory Content Memory chip Address (ptr) e.g., 0xffff 0000 Content (*ptr) e.g., 0xff char *ptr; int *ptr1; *ptr= 0xff; *ptr1= 0x???? ??ff Content (*ptr1) e.g., 0x???? ??ff 26 Initialize a string in C char a[]=“I am great”; char *ptr=“You are great too”; How many bytes are allocated to a? How many bytes are allocated to ptr? 27