Rootfs's Blog

1. <4.1.1> However, to improve the performance of programs, data structures (especially stacks) should be aligned on natural boundaries when-ever possible. The reason for this is that the processor requires two memory accesses to make an unaligned memory access; aligned accesses require only one memory access. Why?

Answer: <http://en.wikipedia.org/wiki/Data_structure_alignment&gt; 2. Problems:

A computer accesses memory by a single memory word at a time. As long as the memory word size is at least as large as the largest primitive data type supported by the computer, aligned accesses will always access a single memory word. This may not be true for misaligned data accesses.

If the highest and lowest bytes in a datum are not within the same memory word the computer must split the datum access into multiple memory accesses. This requires a lot of complex circuitry to generate the memory accesses and coordinate them. To handle the case where the memory words are in different memory pages the processor must either verify that both pages are present before executing the instruction or be able to handle a TLB miss or a page fault on any memory access during the instruction execution.

When a single memory word is accessed the operation is atomic, i.e. the whole memory word is read or written at once and other devices must wait until the read or write operation completes before they can access it. This may not be true for unaligned accesses to multiple memory words, e.g. the first word might be read by one device, both words written by another device and then the second word read by the first device so that the value read is neither the original value nor the updated value. Although such failures are rare, they can be very difficult to identify.

2. <4.2.1.2> two’s complement.

Answer: <http://en.wikipedia.org/wiki/Two’s_complement&gt;

The two’s complement of a binary number is defined as the value obtained by subtracting the number from a large power of two (specifically, from 2^N for an N-bit two’s complement).

In finding the two’s complement of a binary number, the bits are inverted, or “flipped”, by using the bitwise NOT operation; the value of 1 is then added to the resulting value. Bit overflow is ignored, which is the normal case with the zero value.

Note that the two’s complement of zero is zero: inverting gives all ones, and adding one changes the ones back to zeros (the overflow is ignored). Also the two’s complement of the most negative number representable (e.g. a one as the most-significant bit and all other bits zero) is itself.

A more formal definition of a two’s-complement negative number (denoted by N* in this example) is derived from the equation N * = 2ⁿ − N, where N is the corresponding positive number and n is the number of bits in the representation.

3.<4.3> Near pointer and far pointer.

A near pointer is a 32-bit (or 16-bit) offset (also called an effective address) within a segment. Near pointers are used for all memory references in a flat memory model or for references in a segmented model where the identity of the segment being accessed is implied.

A far pointer is a logical address, consisting of a 16-bit segment selector and a 32-bit (or 16-bit) offset. Far pointers are used for memory references in a segmented memory model where the identity of a segment being accessed must be specified explicitly.

4.<4.7>BCD.

Binary-coded decimal integers (BCD integers) are unsigned 4-bit integers with valid values ranging from 0 to 9. IA-32 architecture defines operations on BCD integers located in one or more general-purpose registers or in one or more x87 FPU registers.

When operating on BCD integers in x87 FPU data registers, BCD values are packed in an 80-bit format and referred to as decimal integers. In this format, the first 9 bytes hold 18 BCD digits, 2 digits per byte. The most significant bit of byte 10 contains the sign bit (0 = positive and 1= negative; bits 0 through 6 of byte 10 are don’t care bits). Negative decimal integers are not stored in two’s complement form; they are distinguished from positive decimal integers only by the sign bit.

Please see Table 4-4.  Packed Decimal Integer Encodings