AXI中wrap边界的计算
在AXI中，主要是三种猝发模式，FIXED，INCR，WRAP。前两种比较容易理解，第三种的边界计算如下。
WRAP猝发类似于INCR猝发。在WRAP中，地址将根据SiZE递增，但是达到地址上限时，地址将换到地址下限。
在WRAP地址计算过程中有两个注意事项，
-
计算WRAP的地址上限
-
低位地址回旋到WRAP
WRAP突发有限制：
-
起始地址必须与每次传输的大小对齐
-
突发长度必须为2、4、8或16个传输
计算方式如下：
地址下界：
地址上界：
Ex1：
AxADDR = 0x04, AxLEN = 3, AxSIZE = 2, AxBURST = 3。
Start_Address = 0x04
Burst_Length = 4
Number_Bytes = 2^2=4
因此地址下界
= (0x04/(4x4)) x (4x4)
= 0
地址上界
= 0 + (4 x 4)
= 16
Address_0 = 4
Address_1 = Address_0 + Number_Bytes = 4 + 4 = 8
Address_2 = Address_1 + Number_Bytes = 8 + 4 = 12
Address_3 = Address_2 + Number_Bytes = 12 + 4 = 16
因为上界为16，因此地址Address_3 = 0。
因此地址为4,8,12,0。
Ex2：
AxADDR = 0x38, AxLEN = 3, AxSIZE = 2, AxBURST = 3。
Start_Address = 0x38
Burst_Length = 4
Number_Bytes = 2^2=4
因此地址下界
= (0x38/(4x4)) x (4x4)
= 0x30
地址上界
= 0x30 + (4 x 4)
= 0x40
Address_0 = 0x38
Address_1 = Address_0 + Number_Bytes = 0x38 + 0x4 = 0x3C
Address_2 = Address_1 + Number_Bytes = 0x3C + 0x4 = 0x40
因为Address_2 == 0x40, 达到上边界，因此Address_2 = 0x30
Address_3 = Address_2 + Number_Bytes = 0x30 + 0x4 = 0x34
因此地址为 0x38, 0x3C, 0x30, 0x34。
Ex3：
AxADDR = 0x34, AxLEN = 7, AxSIZE = 2, AxBURST = 3。
Start_Address = 0x34
Burst_Length = 8
Number_Bytes = 2^2=4
因此地址下界
= (0x34/(4x8)) x (4x8)
= 0x20
地址上界
= 0x20 + (4 x 8)
= 0x40
Address_0 = 0x34
Address_1 = Address_0 + Number_Bytes = 0x34 + 4 = 0x38
Address_2 = Address_1 + Number_Bytes = 0x38 + 4 = 0x3C
Address_3 = Address_2 + Number_Bytes = 0x3C + 4 = 0x40
因为Address_3 == 0x40,达到上边界，因此 Address_3 = 0x20
Address_4 = Address_3 + Number_Bytes = 0x20 + 4 = 0x24
Address_5 = Address_4 + Number_Bytes = 0x24 + 4 = 0x28
Address_6 = Address_5 + Number_Bytes = 0x28 + 4 = 0x2C
Address_7 = Address_6 + Number_Bytes = 0x2C + 4 = 0x30
因此地址为 0x34, 0x38, 0x3C, 0x20, 0x24, 0x28, 0x2C, 0x30
