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，

1. 计算WRAP的地址上限
2. 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

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