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Software engineer (entrepreneur and investor at times). These days doing performant frontend and graphics on the web platform at Barcelona Supercomputing Center

The Hw/Sw Interface. Section 1: Memory, Data, and Addressing

30 Nov 2015   |   iker hurtado  
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These are my notes on the section 1 of the course (Coursera): The Hardware/Software Interface (by Gaetano Borriello, Luis Ceze - University of Washington)

Memory organization

I extract some interesting slides of this topic:

The memory is a very large array of bytes, each with an address. Regardless of the machine word size, there are one address per byte in the memory.

Every machine has a word size. It is usually the nominal size of integer-valued data and the size of integers for memory addressing (pointers). So, The addressing capacity in a machine depends on its word size.

To address a word in memory the address of its first byte is used.

For backward compatibility, many CPUs support different word sizes (always a power‐of‐2 in the number of bytes).

Addresses and pointers

Pointer is a data object that contains an address (location in memory).

Byte ordering

How should bytes within multi-byte word be ordered in memory? Response: Endianness (big endian vs. little endian).

Manipulating data in memory using C

We can do arithmetic on pointers: ptr= ptr + 1;

The sentence adds 4 (the type of ptr is int* and an int uses 4 bytes)

Pointer arithmetic can be used for array indexing in C (if pointer and array have the same type, of course). In general: &array[i] is the same as (array + i), which implicitly computes: &array[0] + i*sizeof(array[0]);

Assignment in C

Left-hand-side (LHS) must evaluate to a memory location (a variable). Right-hand-side (LHS) must evaluate to a value (could be an address)

Interesting example: *x = y; The value of y is copied to location to which x points.

Finally, a good practical slide on how to show the data representation of our data in memory:

Boolean bit-­level operations in C

What caught my attention of this part was the sets representation:

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