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POIn C, a pointer to a structure that needs to access a list of byte pointers
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copied!<p>First off, sorry for the title, it was hard to come up with a description. I am attempting to create a structure that will access a peripheral that has quite of few internal registers that are controlled by 3 address lines. So I have the following structure definition:</p> <pre><code>typedef struct { union { unsigned char *reg0aPtr; unsigned char *reg0bPtr; } RegsAddrOffset0; union { unsigned char *reg1aPtr; unsigned char *reg1bPtr; } RegsAddrOffset1; ... union { unsigned char *reg7aPtr; unsigned char *reg7bPtr; } RegsAddrOffset7; } DeviceRegMap; </code></pre> <p>Then if I declare a variable as so</p> <pre><code>DeviceRegMap *uartRegMap = (DeviceRegMap *)(0xD0000000); </code></pre> <p>Now, if I try to to use the uartRegMap variable to access the memory mapped registers I thought I would see that *(uartRegMap->RegsAddrOffset0.reg0aPtr) would read the data from address 0xD0000000, which is does. If I use *(uartRegMap->RegsAddrOffset1.reg1bPtr) I thought I would access 0xD0000001, but it is actually accesses address 0xD0000004. Since the processor is 32-bit the natural alignment is 4-bytes. I am thinking since the size of a pointer is an int which in this case is 32-bits this is the reason why I am accessing on 4-byte address ranges (e.g., 0xD0000000, 0xD0000004, 0xD0000008, etc).</p> <p>If I declared an instance of the DeviceRegMap datatype I would expect the following initialization:</p> <pre><code>DeviceRegMap uartRegMap = { (unsigned char *)0xD0000000, (unsigned char *)0xD0000001, ... (unsigned char *)0xD0000007, }; </code></pre> <p>Then if I wanted to access address 0xD0000007 then I would use *(uartRegMap.RegsAddrOffset7.reg7bPtr)</p> <p>The question is why is the pointer to the DeviceRegMap data type access memory on 4-byte alignments (e.g., 0xD0000000, 0xD0000004, etc) instead of byte alignments (e.g., 0xD0000000, 0xD0000001, etc). With regards to the toolset I am using Wind River diab compiler. Thanks.</p> <p>Mark</p> <p>EDIT: Based upon abelenky's comment</p>

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