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Hi, in this lecture 

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we are going to learn how to get memory map. We still use bios function to do it.

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The service we use is called system map service 

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by interrupt 15, it returns a list of memory blocks to us.

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Each block is 20 bytes.

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The structure of blocks is shown here.

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The first qword is 64-bit physical start address of the memory region. 

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The second qword is length in bytes of the region. The last dword indicates the type of memory with 1 

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being free memory which we can use.  2 being not available to us,  etc. In our project, 

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we only collect memory region of type 1. 

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After we get the free memory information, the memory module will use the information to allocate memory 

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for our system.

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OK, let's get started.

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First off,  we define the label.

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get memory info start.

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We pass e820 to eax 

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and the ascii code for smap to edx. 

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Then we save 20 which is the length of memory block to ecx. 

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Next we save the memory address in which we saved the memory block returned in register edi 

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and ebx should be 0 before we call the function. So we clear ebx using xor instruction.

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At this point, we can call the service through interrupt 15.

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If the carry flag is set for the first call to the function, the service e820 is not available. 

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We simply jump to not support. 

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If it returns the memory info successfully, we continue to retrieve the memory info. 

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So we define another label, get mem info.

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First off, we adjust edi to point to the next memory address 

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to receive the next memory block. Each memory block is 20 bytes, here we add 20 to edi. 

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Then we pass the same parameters to eax, ecx and edx. 

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Note that ebx must be preserved for the next call of the function. So we don’t change ebx value

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.

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Call the service again 

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and test carry flag.

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If carry flag is set this time, it means that we reach the end of memory blocks. 

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So we jump to get memory done 

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we define the label get memory done. 

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And change the message

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to get memory info done.

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If it is not set, 

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we test ebx, 

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if ebx is nonzero, we jump back to label get memory info 

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So here we use jnz instruction, which jump if zero flag is not set.

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When we jump back to get memory info,

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we continue querying the memory info.

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Otherwise, we reach the end of the memory block and we are done.

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OK, let's build our project.

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As you can see, get memory info done message is printed. Before we wrap up this lecture,

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one thing I need to mention is that since they haven't implemented print formatted function such as

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print function in C language, we don't print the detailed information about memory map

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we just retrieved in this lecture.

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If you want to see the details about memory map, you can check section memory management

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and use the image to boot the computer

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and see the memory map info.