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ST20C2 데이터시트(PDF) 9 Page - STMicroelectronics |
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ST20C2 데이터시트(HTML) 9 Page - STMicroelectronics |
9 / 212 page 9/212 ® Subscripts are used to indicate particular bits in a word; e.g. Aregi for bit i of Areg; and Areg0..7 for the least significant byte of Areg. Note that bit 0 is the least significant bit in a word, and bit 31 is the most significant bit. Generally, if the description does not mention the state of a register or memory location after the instruction, then the value will not be changed by the instruction. One exception to this general rule is Iptr, which is assigned the address of the next instruction in the code before every instruction execution starts. The Iptr is included in the description only when it is directly affected by the instruction (e.g. in the jump instruction). In these cases the address of the next instruction is indicated by the comment “ next instruction”. Scheduling operations Some registers, such as the timer and scheduling list pointers and some special workspace locations, can be changed at any time by scheduling operations. Changes to these are included in the description only when they are directly caused by the instruction, and not just as an effect of any scheduling operation which might take place. 1.7.3 Undefined v alues Many instructions leave the contents of a register or memory location in an undefined state. This means that the value of the location may be changed by the instruction, but the new value cannot be easily defined, or is not a meaningful result of the instruction. For example, when the integer stack is popped, Creg becomes undefined, i.e. it does not contain any meaningful data. An undefined value is represented by the name undefined. The values of registers which become undefined as a result of executing an instruction are implementation dependent and are not guaranteed to be the same on different members of the ST20 family of processors. 1.7.4 Data types The instruction set includes operations on four sizes of data: 8, 16, 32 and 64-bit objects. 8-bit and 16-bit data can represent signed or unsigned integers; 32-bit data can represent addresses, signed or unsigned integers, or single length floating point numbers; and 64-bit data can represent signed or unsigned integers, or double length floating point v alues. Normally it is clear from the context (e.g. from the operators used) whether a particular object represents a signed, unsigned or floating point number. A subscripted label is added (e.g. Aregunsigned) to clarify where necessary. 1.7.5 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 four 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 object in memory |
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