AMBA AHB协议学习过程中遇到的问题

姬谦正

最近在学习AMBA AHB协议，遇到了一些问题自己不是很理解：1.在突发传输部分提到了一句话 All transfers within a burst must be aligned to the address boundary equal to the size of the transfer.For example , word transfers must be aligned to word address boundaries(that is A[1:0] = 00),halfword transfers must be aligned to halfword address boundaries(that is A[0] = 0).首先不是很理解地址边界对齐的作用，是为了保证每次传输的数据是一个正确的数据吗？
2.wrapping bursts 部分
For wrapping bursts, if the start address of the transfer is not aligned to the total number of bytes in the burst (size x beats) then the address of the transfers in the burst will wrap when the boundary is reached. For example, a four-beat wrapping burst of word (4-byte) accesses will wrap at 16-byte boundaries. Therefore, if the start address of the transfer is 0x34, then it consists of four transfers to addresses 0x34, 0x38, 0x3C and 0x30.

这里的16-byte地址边界是指，无论0x34, 0x38, 0x3C and 0x30中的哪个是开始的地址，最终回环突发传输的地址都包括这四个吗？
3.Every slave must have a predetermined maximum number of wait states that it will insert before it backs off the bus, in order to allow the calculation of the latency of accessing the bus.这里为什么要计算访问总线的延迟

mabin99

1，地址对齐就是如果传输的是word，4个byte，那么每次地址都是加4，如果传输的是一个halfword，那么每次地址就是加2，如果传输的是一个byte，那么每次地址就是加1；
2，WRAP类型得看是wrap几，如果以wrap8 word为例，那么地址默认以8个word进行切分，每次的访问都固定在这8个word里，如果地址是0-7，从4开始访问，那么顺序就是45670123，以此类推；