I.D special Cycles
Frequently, continuous memory places must be moved over the bus. Relocating the deal with once and also incrementing a counter on the storage are much more efficient than sending the resolve each time, particularly if the bus is multiplexed. Block transfers space supported by the most complicated buses and are accessible with constraints on the many recent 32-bit microprocessors.
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Reading from plenty of devices simultaneously is referred to as broadcall. This costs extr bus lines and also transfer time and is not frequently implemented.
Combined cycles room possible. Read–modify–write cycles are interesting in multiprocessor systems. Read-after-write cycles are never ever implemented because one has confidence in what has actually been written.
Finally, break-up cycles space a means of having actually only create cycles in the system. In stimulate to review some information, one write a read request and waits. The slave interrupts the grasp to acquire its information read, and also the process may continue. This is useful when the slave, for example, a disc interface, transfers blocks of data and also is slow: It would be inefficient to wait because that an acknowledge.
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Joseph Yiu, in The Definitive guide to the eight Cortex-M3 (Second Edition), 2010
5.2 memory Maps
The Cortex-M3 processor has a solved memory map (see figure 5.1). This renders it much easier to port software program from one Cortex-M3 product to another. Because that example, components described in vault sections, such together Nested Vectored Interrupt Controller (NVIC) and also Memory security Unit (MPU), have the same memory areas in all Cortex-M3 products. Nevertheless, the memory map meaning allows great flexibility so that manufacturers can differentiate their Cortex-M3-based product indigenous others.
The details of these contents are debated in later chapters top top debugging features.
The Cortex-M3 processor has a total of 4 GB of address space. Program code can be situated in the password region, the revolution Random access Memory (SRAM) region, or the external RAM region. However, that is best to put the program code in the code region because v this arrangement, the indict fetches and also data accesses are lugged out simultaneously on two different bus interfaces.
The SRAM memory selection is for connecting internal SRAM. Access to this an ar is carried out via the system user interface bus. In this region, a 32-MB range is defined as a bit-band alias. In ~ the 32-bit‑band alias storage range, each word resolve represents a solitary bit in the 1-MB bit-band region. A data write access to this bit-band alias memory selection will be convert to an atomic READ-MODIFY-WRITE procedure to the bit-band region so as to permit a regime to set or clear individual data bits in the memory. The bit-band operation applies only come data accesses no instruction fetches. By putting Boolean details (single bits) in the bit-band region, we can pack many Boolean data in a single word while still enabling them come be available individually via bit-band alias, therefore saving memory room without the need for dealing with READ-MODIFY-WRITE in software. More details ~ above bit-band alias deserve to be uncovered later in this chapter.
Another 0.5-GB block the address variety is allocated come on-chip peripherals. Similar to the SRAM region, this region supports bit-band alias and is accessed via the device bus interface. However, instruction execution in this an ar is not allowed. The bit-band assistance in the peripheral an ar makes it simple to accessibility or change control and status bits of peripherals, making it simpler to program peripheral control.
Two slot of 1-GB memory room are allocated for outside RAM and also external devices. The difference between the two is that routine execution in the external maker region is no allowed, and there are some differences with the caching behaviors.
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The critical 0.5-GB memory is for the system-level components, interior peripheral buses, external peripheral bus, and also vendor-specific system peripherals. There are two segments of the personal peripheral bus (PPB):•
Advanced High-Performance Bus (AHB) PPB, for Cortex-M3 inner AHB peripherals only; this contains NVIC, FPB, DWT, and ITM•
Advance Peripheral Bus (APB) PPB, because that Cortex-M3 internal APB devices and external peripherals (external come the Cortex-M3 processor); the Cortex-M3 allows chip vendors to add additional on-chip APB peripherals on this personal peripheral bus via an APB interface
The NVIC is situated in a memory region called the mechanism control an are (SCS) (see figure 5.2). Besides providing interrupt regulate features, this an ar also provides the manage registers because that SYSTICK, MPU, and also code debugging control.