Ada记录表示 [英] Ada record representation

问题描述

我的记录类型如下：

type Rec_T is record
   a : Bit_12_T;
   b : Bit_4_T;
end record;

其中 Bit_12_T是mod 2 ** 12 Bit_4_T是mod 2 ** 4 。

为了告诉编译器该记录的精确对齐，我使用了 for use record 语句。但是，我想在字节之间分割 a 字段，所以我尝试如下操作：

To tell the compiler the precise alignment of this record, I use the the for use record statement. However, I want to split the a field bewteen the bytes, so I try to do it as follows:

for Rec_T use record
   a at 0 range 0 .. 7; -- The 1st byte
   a at 1 range 0 .. 3; -- The 2nd byte
   b at 1 range 4 .. 7;
end record;

很显然，这不是这样做的方法，因为编译器抱怨以前给出了component子句在行...。

Clearly, this is not the way to do it as the compiler complains that "component clause is previously given at line ...".

问题：是否可以在字节之间拆分一个组件，并且该怎么做？如果不可能，我是否应该使用a_high和a_low，然后使用一些位运算将它们合并在一起？

Question: Is it possible to have a component split between the bytes and how to do it? If it is not possible, should I have a_high and a_low and then use some bit operations to merge them together?

推荐答案

认为该位置是位数组，而不是字节序列。因此，这应该起作用：

Think of the location as an array of bits, not as a sequence of bytes. Hence this should work:

for Rec_T use record
   a at 0 range 0 .. 11; -- bits 0..7 of the 1st byte and bits 0..3 of the 2nd byte
   b at 0 range 12 .. 15; -- bits 4..7 of the second byte
end record;
for Rec_T'Size use 16;

有关更多信息，请参见此处的文档，特别是页面结尾处的示例，也显示了类似的情况。

For more information see documentation here, specifically the example at then end of the page, that shows a similar situation.

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