What Is the Maximum Number of Different Memory Addresses That the 68hc12 Can Read?

Reference to a specific memory location

In a reckoner using virtual retentiveness, accessing the location corresponding to a memory address may involve many levels.

In computing, a retentiveness accost is a reference to a specific retentivity location used at various levels past software and hardware. Retentivity addresses are fixed-length sequences of digits conventionally displayed and manipulated as unsigned integers. Such numerical semantic bases itself upon features of CPU (such as the instruction pointer and incremental address registers), besides upon utilize of the memory like an array endorsed by various programming languages.

Types [edit]

Physical addresses [edit]

A digital reckoner's main memory consists of many memory locations. Each memory location has a physical address which is a code. The CPU (or other device) can use the lawmaking to access the corresponding retentiveness location. More often than not only arrangement software, i.e. the BIOS, operating systems, and some specialized utility programs (east.one thousand., retention testers), accost physical retentiveness using car code operands or processor registers, instructing the CPU to direct a hardware device, called the retention controller, to use the memory bus or system bus, or divide control, address and information busses, to execute the program'southward commands. The retentivity controllers' bus consists of a number of parallel lines, each represented by a binary digit (bit). The width of the passenger vehicle, and thus the number of addressable storage units, and the number of bits in each unit of measurement, varies among computers.

Logical addresses [edit]

A calculator program uses retention addresses to execute machine code, and to store and call back information. In early on computers logical and physical addresses corresponded, but since the introduction of virtual memory most application programs do not accept a knowledge of physical addresses. Rather, they accost logical addresses, or virtual addresses, using the computer's retentiveness direction unit of measurement and operating arrangement memory mapping; see below.

Unit of measurement of accost resolution [edit]

Near modern computers are byte-addressable. Each address identifies a single byte (eight bits) of storage. Data larger than a unmarried byte may exist stored in a sequence of consecutive addresses. There exist word-addressable computers, where the minimal addressable storage unit is exactly the processor'southward word. For example, the Information General Nova minicomputer, and the Texas Instruments TMS9900 and National Semiconductor IMP-16 microcomputers used xvi bit words, and there were many 36-scrap mainframe computers (due east.g., PDP-x) which used xviii-bit word addressing, not byte addressing, giving an address space of ii¹⁸ 36-bit words, approximately one megabyte of storage. The efficiency of addressing of memory depends on the flake size of the bus used for addresses – the more bits used, the more addresses are available to the figurer. For instance, an 8-scrap-byte-addressable motorcar with a twenty-bit address autobus (eastward.g. Intel 8086) can accost ii²⁰ (1,048,576) retentiveness locations, or one MiB of retentiveness, while a 32-bit bus (e.thou. Intel 80386) addresses 2³² (4,294,967,296) locations, or a 4 GiB address infinite. In contrast, a 36-bit word-addressable machine with an eighteen-bit accost bus addresses only 2¹⁸ (262,144) 36-flake locations (9,437,184 $.25), equivalent to 1,179,648 8-bit bytes, or 1152 KiB, or 1.125 MiB — slightly more than than the 8086.

Some older computers (decimal computers), were decimal digit-addressable. For example, each address in the IBM 1620's magnetic-core memory identified a single 6 scrap binary-coded decimal digit, consisting of a parity bit, flag chip and four numerical $.25. The 1620 used 5-digit decimal addresses, so in theory the highest possible address was 99,999. In exercise, the CPU supported twenty,000 retention locations, and up to 2 optional external retention units could be added, each supporting twenty,000 addresses, for a total of threescore,000 (00000–59999).

Word size versus address size [edit]

Word size is a characteristic of computer architecture denoting the number of bits that a CPU tin can process at i time. Modern processors, including embedded systems, normally have a discussion size of 8, xvi, 24, 32 or 64 bits; virtually current general-purpose computers use 32 or 64 bits. Many different sizes have been used historically, including 8, 9, 10, 12, 18, 24, 36, 39, xl, 48 and 60 bits.

Very often, when referring to the word size of a modernistic computer, 1 is also describing the size of address space on that computer. For instance, a computer said to be "32-bit" also unremarkably allows 32-chip memory addresses; a byte-addressable 32-fleck computer tin address 2³² = 4,294,967,296 bytes of memory, or 4 gibibytes (GiB). This allows ane memory address to be efficiently stored in one word.

However, this does non ever concord true. Computers tin take memory addresses larger or smaller than their word size. For instance, many eight-bit processors, such equally the MOS Engineering 6502, supported 16-bit addresses— if not, they would take been limited to a mere 256 bytes of memory addressing. The 16-bit Intel 8088 and Intel 8086 supported 20-bit addressing via partitioning, allowing them to access ane MiB rather than 64 KiB of retentiveness. All Intel Pentium processors since the Pentium Pro include Physical Accost Extensions (PAE) which support mapping 36-fleck physical addresses to 32-bit virtual addresses. Many early processors held 2 addresses per discussion , such as 36-bit processors.

In theory, modern byte-addressable 64-flake computers can accost 2⁶⁴ bytes (16 exbibytes), but in practise the amount of memory is limited past the CPU, the memory controller, or the printed circuit board blueprint (e.k., number of physical memory connectors or amount of soldered-on memory).

Contents of each memory location [edit]

Each memory location in a stored-program computer holds a binary number or decimal number of some sort. Its interpretation, as data of some data blazon or as an instruction, and use are determined by the instructions which retrieve and dispense it.

Some early programmers combined instructions and information in words equally a manner to salve memory, when it was expensive: The Manchester Marking 1 had infinite in its 40-bit words to shop little $.25 of data – its processor ignored a small section in the heart of a word – and that was ofttimes exploited as extra data storage.^{[ citation needed ]} Cocky-replicating programs such as viruses care for themselves sometimes as data and sometimes every bit instructions. Self-modifying lawmaking is generally deprecated nowadays, as it makes testing and maintenance disproportionally difficult to the saving of a few bytes, and can also give incorrect results considering of the compiler or processor's assumptions near the machine's state, but is even so sometimes used deliberately, with smashing care.

Accost space in application programming [edit]

In modernistic multitasking surroundings, an awarding process usually has in its address infinite (or spaces) chunks of memory of following types:

• Machine code, including:
  • programme's own code (historically known equally code segment or text segment);
  • shared libraries.
• Data, including:
  • initialized data (data segment);
  • uninitialized (but allocated) variables;
  • run-time stack;
  • heap;
  • shared retention and memory mapped files.

Some parts of address infinite may be non mapped at all.

Some systems have a "separate" retentiveness compages where machine code, constants, and data are in different locations, and may have different address sized. For instance, PIC18 microcontrollers have a 21-bit program counter to accost machine lawmaking and constants in Flash memory, and 12-bit address registers to accost information in SRAM.

Addressing schemes [edit]

A computer program can access an address given explicitly – in low-level programming this is normally called an absolute address , or sometimes a specific accost, and is known as pointer information type in college-level languages. Just a program tin besides utilize relative accost which specifies a location in relation to somewhere else (the base of operations accost). There are many more indirect addressing modes.

Mapping logical addresses to physical and virtual memory also adds several levels of indirection; see beneath.

Memory models [edit]

Many programmers prefer to address memory such that there is no distinction betwixt code infinite and data space (cf. higher up), as well as from physical and virtual retentiveness (see beneath) — in other words, numerically identical pointers refer to exactly the same byte of RAM.

Even so, many early computers did not support such a apartment memory model — in item, Harvard architecture machines force plan storage to be completely split from data storage. Many modern DSPs (such as the Motorola 56000) have three separate storage areas — plan storage, coefficient storage, and data storage. Some commonly used instructions fetch from all 3 areas simultaneously — fewer storage areas (even if there were the aforementioned total bytes of storage) would make those instructions run slower.

Retentivity models in x86 compages [edit]

Early x86 computers apply the segmented retention model addresses based on a combination of ii numbers: a memory segment, and an beginning within that segment.

Some segments are implicitly treated as lawmaking segments, dedicated for instructions, stack segments, or normal data segments. Although the usages are different, the segments do not have unlike memory protections reflecting this. In the flat memory model all segments (segment registers) are generally fix to zip, and only offsets are variable.

See as well [edit]

• Memory model (programming)
• Retentivity allocation
• Retentivity accost register
• Base of operations address
• Showtime (reckoner science), also known as a displacement
• Endianness
• Retention management unit (MMU)
• Page table
• Retention protection
• Memory segmentation
• Low-level programming linguistic communication

References [edit]

What Is the Maximum Number of Different Memory Addresses That the 68hc12 Can Read?

Source: https://en.wikipedia.org/wiki/Memory_address

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