This glossary describes some of the terms used in technical documents from ARM.
- Advanced eXtensible Interface (AXI)
A bus protocol that supports separate address/control and data phases, unaligned data transfers using byte strobes, burst-based transactions with only start address issued, separate read and write data channels to enable low-cost DMA, ability to issue multiple outstanding addresses, out-of-order transaction completion, and easy addition of register stages to provide timing closure.
The AXI protocol also includes optional extensions to cover signaling for low-power operation.
AXI is targeted at high performance, high clock frequency system designs and includes a number of features that make it very suitable for high speed sub-micron interconnect.
- Advanced Microcontroller Bus Architecture (AMBA)
A family of protocol specifications that describe a strategy for the interconnect. AMBA is the ARM open standard for on-chip buses. It is an on-chip bus specification that describes a strategy for the interconnection and management of functional blocks that make up a System-on-Chip (SoC). It aids in the development of embedded processors with one or more CPUs or signal processors and multiple peripherals. AMBA complements a reusable design methodology by defining a common backbone for SoC modules.
- Advanced Peripheral Bus (APB)
A simpler bus protocol than AXI and AHB. It is designed for use with ancillary or general-purpose peripherals such as timers, interrupt controllers, UARTs, and I/O ports. Connection to the main system bus is through a system-to-peripheral bus bridge that helps to reduce system power consumption.
See Advanced Microcontroller Bus Architecture.
See Advanced Peripheral Bus.
See Automatic Test Pattern Generation.
- Automatic Test Pattern Generation (ATPG)
The process of automatically generating manufacturing test vectors for an ASIC design, using a specialized software tool.
See Advanced eXtensible Interface.
- AXI channel order and interfaces
The block diagram shows:
the order in which AXI channel signals are described
the master and slave interface conventions for AXI components.
- AXI terminology
The following AXI terms are general. They apply to both masters and slaves:
- Active read transaction
A transaction for which the read address has transferred, but the last read data has not yet transferred.
- Active transfer
A transfer for which the xVALID handshake has asserted, but for which xREADY has not yet asserted.
- Active write transaction
A transaction for which the write address or leading write data has transferred, but the write response has not yet transferred.
- Completed transfer
A transfer for which the xVALID/xREADY handshake is complete.
The non-handshake signals in a transfer.
An entire burst of transfers, comprising an address, one or more data transfers and a response transfer (writes only).
An initiator driving the payload and asserting the relevant xVALID signal.
A single exchange of information. That is, with one xVALID/xREADY handshake.
The following AXI terms are master interface attributes. To obtain optimum performance, they must be specified for all components with an AXI master interface:
- Combined issuing capability
The maximum number of active transactions that a master interface can generate. It is specified for master interfaces that use combined storage for active write and read transactions. If not specified then it is assumed to be equal to the sum of the write and read issuing capabilities.
- Read ID capability
The maximum number of different ARID values that a master interface can generate for all active read transactions at any one time.
- Read ID width
The number of bits in the ARID bus.
- Read issuing capability
The maximum number of active read transactions that a master interface can generate.
- Write ID capability
The maximum number of different AWID values that a master interface can generate for all active write transactions at any one time.
- Write ID width
The number of bits in the AWID and WID buses.
- Write interleave capability
The number of active write transactions for which the master interface is capable of transmitting data. This is counted from the earliest transaction.
- Write issuing capability
The following AXI terms are slave interface attributes. To obtain optimum performance, they must be specified for all components with an AXI slave interface:
- Combined acceptance capability
The maximum number of active transactions that a slave interface can accept. It is specified for slave interfaces that use combined storage for active write and read transactions. If not specified then it is assumed to be equal to the sum of the write and read acceptance capabilities.
- Read acceptance capability
The maximum number of active read transactions that a slave interface can accept.
- Read data reordering depth
The number of active read transactions for which a slave interface can transmit data. This is counted from the earliest transaction.
- Write acceptance capability
The maximum number of active write transactions that a slave interface can accept.
- Write interleave depth
The number of active write transactions for which the slave interface can receive data. This is counted from the earliest transaction.
Alternative word for an individual transfer within a burst. For example, an INCR4 burst comprises four beats.
See Also Burst.
Byte ordering scheme in which bytes of decreasing significance in a data word are stored at increasing addresses in memory.
See Also Little-endian and Endianness.
A group of transfers to consecutive addresses. Because the addresses are consecutive, there is no requirement to supply an address for any of the transfers after the first one. This increases the speed at which the group of transfers can occur. Bursts over AMBA are controlled using signals to indicate the length of the burst and how the addresses are incremented.
See Also Beat.
- Clock gating
Gating a clock signal for a macrocell with a control signal and using the modified clock that results to control the operating state of the macrocell.
A 64-bit data item. The contents are taken as being an unsigned integer unless otherwise stated.
Byte ordering. The scheme that determines the order that successive bytes of a data word are stored in memory. An aspect of the system’s memory mapping.
See Also Little-endian and Big-endian.
A 16-bit data item.
Byte ordering scheme in which bytes of increasing significance in a data word are stored at increasing addresses in memory.
See Also Big-endian and Endianness.
A complex logic block with a defined interface and behavior. A typical VLSI system comprises several macrocells (such as a processor, an ETM, and a memory block) plus application-specific logic.
A field in a control register or instruction format is reserved if the field is to be defined by the implementation, or produces Unpredictable results if the contents of the field are not zero. These fields are reserved for use in future extensions of the architecture or are implementation-specific. All reserved bits not used by the implementation must be written as 0 and read as 0.
A data item stored at an address that is not divisible by the number of bytes that defines the data size is said to be unaligned. For example, a word stored at an address that is not divisible by four.
For reads, the data returned when reading from this location is unpredictable. It can have any value. For writes, writing to this location causes unpredictable behavior, or an unpredictable change in device configuration. Unpredictable instructions must not halt or hang the processor, or any part of the system.
A 32-bit data item.
 The letter x in the signal name denotes an AXI channel as follows:
Write address channel.
Write data channel.
Write response channel.
Read address channel.
Read data channel.