Architecture

The Name For A Memory Location That May Hold Data Is:, Memory Location

J.D. Nicoud, in Encyclopedia of Physical Science and Technology (Third Edition), 2003

I.D Special Cycles

Frequently, consecutive memory locations must be transferred over the bus. Moving the address once and incrementing a counter on the memory are more efficient than sending the address each time, especially if the bus is multiplexed. Block transfers are supported by the most complex buses and are available with limitations on the most recent 32-bit microprocessors.

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Reading from many devices simultaneously is called broadcall. This costs additional bus lines and transfer time and is not frequently implemented.

Combined cycles are possible. Read–modify–write cycles are interesting in multiprocessor systems. Read-after-write cycles are never implemented because one has confidence in what has been written.

Finally, split cycles are a way of having only write cycles in the system. In order to read some information, one writes a read request and waits. The slave interrupts the master to get its information read, and the process may continue. This is useful when the slave, for example, a disk interface, transfers blocks of data and is slow: It would be inefficient to wait for an acknowledge.

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Joseph Yiu, in The Definitive Guide to the ARM Cortex-M3 (Second Edition), 2010

5.2 Memory Maps

The Cortex-M3 processor has a fixed memory map (see Figure 5.1). This makes it easier to port software from one Cortex-M3 product to another. For example, components described in previous sections, such as Nested Vectored Interrupt Controller (NVIC) and Memory Protection Unit (MPU), have the same memory locations in all Cortex-M3 products. Nevertheless, the memory map definition allows great flexibility so that manufacturers can differentiate their Cortex-M3-based product from others.

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The details of these components are discussed in later chapters on debugging features.

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The Cortex-M3 processor has a total of 4 GB of address space. Program code can be located in the code region, the Static Random Access Memory (SRAM) region, or the external RAM region. However, it is best to put the program code in the code region because with this arrangement, the instruction fetches and data accesses are carried out simultaneously on two separate bus interfaces.

The SRAM memory range is for connecting internal SRAM. Access to this region is carried out via the system interface bus. In this region, a 32-MB range is defined as a bit-band alias. Within the 32-bit‑band alias memory range, each word address represents a single bit in the 1-MB bit-band region. A data write access to this bit-band alias memory range will be converted to an atomic READ-MODIFY-WRITE operation to the bit-band region so as to allow a program to set or clear individual data bits in the memory. The bit-band operation applies only to data accesses not instruction fetches. By putting Boolean information (single bits) in the bit-band region, we can pack multiple Boolean data in a single word while still allowing them to be accessible individually via bit-band alias, thus saving memory space without the need for handling READ-MODIFY-WRITE in software. More details on bit-band alias can be found later in this chapter.

Another 0.5-GB block of address range is allocated to on-chip peripherals. Similar to the SRAM region, this region supports bit-band alias and is accessed via the system bus interface. However, instruction execution in this region is not allowed. The bit-band support in the peripheral region makes it easy to access or change control and status bits of peripherals, making it easier to program peripheral control.

Two slots of 1-GB memory space are allocated for external RAM and external devices. The difference between the two is that program execution in the external device region is not allowed, and there are some differences with the caching behaviors.

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The last 0.5-GB memory is for the system-level components, internal peripheral buses, external peripheral bus, and vendor-specific system peripherals. There are two segments of the private peripheral bus (PPB):

Advanced High-Performance Bus (AHB) PPB, for Cortex-M3 internal AHB peripherals only; this includes NVIC, FPB, DWT, and ITM

Advance Peripheral Bus (APB) PPB, for Cortex-M3 internal APB devices as well as external peripherals (external to the Cortex-M3 processor); the Cortex-M3 allows chip vendors to add additional on-chip APB peripherals on this private peripheral bus via an APB interface

The NVIC is located in a memory region called the system control space (SCS) (see Figure 5.2). Besides providing interrupt control features, this region also provides the control registers for SYSTICK, MPU, and code debugging control.

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