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ST62T08CB1 Datasheet(PDF) 56 Page - STMicroelectronics |
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ST62T08CB1 Datasheet(HTML) 56 Page - STMicroelectronics |
56 / 104 page ST6208C/ST6209C/ST6210C/ST6220C 56/104 9 INSTRUCTION SET 9.1 ST6 ARCHITECTURE The ST6 architecture has been designed for max- imum efficiency while keeping byte usage to a minimum; in short, to provide byte-efficient pro- gramming. The ST6 core has the ability to set or clear any register or RAM location bit in Data space using a single instruction. Furthermore, pro- grams can branch to a selected address depend- ing on the status of any bit in Data space. 9.2 ADDRESSING MODES The ST6 has nine addressing modes, which are described in the following paragraphs. Three dif- ferent address spaces are available: Program space, Data space, and Stack space. Program space contains the instructions which are to be ex- ecuted, plus the data for immediate mode instruc- tions. Data space contains the Accumulator, the X, Y, V and W registers, peripheral and Input/Output registers, the RAM locations and Data ROM loca- tions (for storage of tables and constants). Stack space contains six 12-bit RAM cells used to stack the return addresses for subroutines and inter- rupts. Immediate. In immediate addressing mode, the operand of the instruction follows the opcode loca- tion. As the operand is a ROM byte, the immediate addressing mode is used to access constants which do not change during program execution (e.g., a constant used to initialize a loop counter). Direct. In direct addressing mode, the address of the byte which is processed by the instruction is stored in the location which follows the opcode. Di- rect addressing allows the user to directly address the 256 bytes in Data Space memory with a single two-byte instruction. Short Direct. The core can address the four RAM registers X, Y, V, W (locations 80h, 81h, 82h, 83h) in short-direct addressing mode. In this case, the instruction is only one byte and the selection of the location to be processed is contained in the op- code. Short direct addressing is a subset of direct addressing mode. (Note that 80h and 81h are also indirect registers). Extended. In extended addressing mode, the 12- bit address needed to define the instruction is ob- tained by concatenating the four least significant bits of the opcode with the byte following the op- code. The instructions (JP, CALL) which use ex- tended addressing mode are able to branch to any address in the 4 Kbyte Program space. Extended addressing mode instructions are two bytes long. Program Counter Relative. Relative addressing mode is only used in conditional branch instruc- tions. The instruction is used to perform a test and, if the condition is true, a branch with a span of -15 to +16 locations next to the address of the relative instruction. If the condition is not true, the instruc- tion which follows the relative instruction is execut- ed. Relative addressing mode instructions are one byte long. The opcode is obtained by adding the three most significant bits which characterize the test condition, one bit which determines whether it is a forward branch (when it is 0) or backward branch (when it is 1) and the four least significant bits which give the span of the branch (0h to Fh) which must be added or subtracted from the ad- dress of the relative instruction to obtain the branch destination address. Bit Direct. In bit direct addressing mode, the bit to be set or cleared is part of the opcode, and the byte following the opcode points to the address of the byte in which the specified bit must be set or cleared. Thus, any bit in the 256 locations of Data space memory can be set or cleared. Bit Test & Branch. Bit test and branch addressing mode is a combination of direct addressing and relative addressing. Bit test and branch instruc- tions are three bytes long. The bit identification and the test condition are included in the opcode byte. The address of the byte to be tested is given in the next byte. The third byte is the jump dis- placement, which is in the range of -127 to +128. This displacement can be determined using a la- bel, which is converted by the assembler. Indirect. In indirect addressing mode, the byte processed by the register-indirect instruction is at the address pointed to by the content of one of the indirect registers, X or Y (80h, 81h). The indirect register is selected by bit 4 of the opcode. Register indirect instructions are one byte long. Inherent. In inherent addressing mode, all the in- formation necessary for executing the instruction is contained in the opcode. These instructions are one byte long. 1 |
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