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전자부품 데이터시트 검색엔진 |
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전자부품 데이터시트 검색엔진 |
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ST20C4 데이터시트(PDF) 10 Page - STMicroelectronics |
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ST20C4 데이터시트(HTML) 10 Page - STMicroelectronics |
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10 / 212 page  ® 10/212 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[]), and that after the instruction if the function is used with a prime (e.g. word ′[]). 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: Areg ′← 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, e.g. writing a byte at address 0 will modify the least significant byte of the word at address 0. Reading and writing in this way cannot be used to access on-chip peripherals. Reading or writing to memory addresses between PeripheralStart and PeripheralEnd will have undefined effects. Data alignment Each of these 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 and 64-bit objects must be word aligned, i.e. the 2 least significant bits of the address must 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, words or double words depending on the data type being accessed. In order to calculate the address of the data, the 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 in the particular units being used. So, for example, a word offset is converted to a byte offset by multiplying it by the number of bytes in a word (4 in the case of the ST20). 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 represent an address which is offset by x words (4x bytes) from register. For example, in the specification of load non-local there is: Areg ′← word[Areg @ n] Here, Areg is loaded with the contents of the word that is n words from the address pointed to by Areg (i.e. Areg + 4n). In all cases, if the given base address has the correct alignment then any offset used will also give a correctly aligned address. |
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