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ST20-C1 Arkusz danych(PDF) 10 Page - STMicroelectronics |
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ST20-C1 Arkusz danych(HTML) 10 Page - STMicroelectronics |
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10 / 205 page  2.2 Instruction definitions 10/205 ® represent addresses, signed or unsigned integers. Generally the arithmetic in signed. In some cases it is clear from the context (e.g. from the operators used) whether a particular object represents a signed or unsigned number. A subscripted label is added (e.g. Areg unsigned) to clarify where necessary. 2.2.5 Representing memory The memory is represented by arrays of each data type. These are indexed by a value representing a byte address. Access to the three data types is represented in the instruction descriptions in the following way: byte[ address]references a byte in memory at the given address sixteen[ address]references a 16-bit half word in memory word[ address]references a 32-bit word in memory For all of these, the state of the machine referenced is that before the instruction if the function is used without a prime (e.g. word[ address]), and that after the instruction if the function is used with a prime (e.g. word ′[address]). For example, writing a value given by an expression, expr, to the word in memory at address addr is represented by: word ′[addr] ← expr and reading a word from a memory location is achieved by: register ′← word[addr] Writing to memory in any of these ways will update the contents of memory, and these updates will be consistently visible to the other representations of the memory. For example, writing a byte at address 0 will modify the least significant byte of the word at address 0. Data alignment Generally, word and half word data items have restrictions on their alignment in memory. Byte values can be accessed at any byte address, i.e. they are byte aligned. 16-bit objects can only be accessed at even byte addresses, i.e. the least significant bit of the address must be 0. 32-bit objects must be word aligned, i.e. the 2 least significant bits of the address m ust be zero. Address calculation An address identifies a par ticular byte in memory. Addresses are frequently calculated from a base address and an offset. For different instructions the offset may be given in units of bytes or words depending on the data type being accessed. In order to calculate the address of the data, a word offset must be converted to a byte offset before being added to the base address. This is done by multiplying the offset by the number of bytes per word, i.e. 4. As there are many accesses to memory at word offsets, a shorthand notation is used to represent the calculation of a word address. The notation register @ x is used to |
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