CPSC 2310 - DAY 14
SEPTEMBER 30, 2016

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MULTIPLICATION
--------------

Convert the constant into a sum of powers of two (can allow a subtract)

convert the multiplications by powers of two into left shifts

57x = 32x +16x +8x + x

(x<<5) + (x<<4) + (x<<3) + x

 ^5 is from 2^5

shift by 2 powers

DATA IN MEMORY
--------------

variables have multiple attributes


	 -
 	 | symbolic name
	 | data type
	 | storage class
variable-| address in memory
	 | byte order for multibyte variables
	 | initialization (optional)
	 | current value
	 -

DATA SIZES
----------

BYTE - 		8 BITS = 1 BYTE - CHAR
HALFWORD - 	16 BITS = 2 BYTES - SHORT
WORD-		32 BITS = 4 BYTES - INT
DOUBLEWORD-	64 BITS = 8 BYTES - DOUBLE/LONG

Alignment in byte-addressable memory

memory access is typically on word-by-word basis (aligned)
(original meaning of "memory word" was the unit of transfer to/from memory)

alignment restrictions on load/store instructions prevent multiple-byte units 
from spanning across two memory words, thus allowing individual load or 
store instructions to make one and only one memory access --
which makes hardware easier to build.

UNALIGNED ACCESS
----------------

may be illegal (system may print "Bus error (core dumped)"

may cause a trap into the operating system or requires extra shifting network
hardware and extra time to perform two memory access and the necessary insertion
of bytes (ie to merge bytes from two memory words into the single-word-length
CPU register on a load or to distribute the bytes from a register into 
two different memory words.

ALLIGNMENT RULES
----------------

Bytes can start anywhere

halfwords start on a halfword boundry

words start on a word boundry

doublewords start on a doubleword boundry