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 MICROCHIP
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55 page 
© 2007 Microchip Technology Inc. DS39605F-page 53 PIC18F1220/1320 5.12 Indirect Addressing, INDF and FSR Registers Indirect addressing is a mode of addressing data mem- ory, where the data memory address in the instruction is not fixed. An FSR register is used as a pointer to the data memory location that is to be read or written. Since this pointer is in RAM, the contents can be modified by the program. This can be useful for data tables in the data memory and for software stacks. Figure 5-8 shows how the fetched instruction is modified prior to being executed. Indirect addressing is possible by using one of the INDF registers. Any instruction, using the INDF regis- ter, actually accesses the register pointed to by the File Select Register, FSR. Reading the INDF register itself, indirectly (FSR = 0), will read 00h. Writing to the INDF register indirectly, results in a no operation (NOP). The FSR register contains a 12-bit address, which is shown in Figure 5-9. The INDFn register is not a physical register. Address- ing INDFn actually addresses the register whose address is contained in the FSRn register (FSRn is a pointer). This is indirect addressing. Example 5-5 shows a simple use of indirect addressing to clear the RAM in Bank 1 (locations 100h-1FFh) in a minimum number of instructions. EXAMPLE 5-5: HOW TO CLEAR RAM (BANK 1) USING INDIRECT ADDRESSING There are three indirect addressing registers. To address the entire data memory space (4096 bytes), these registers are 12-bit wide. To store the 12 bits of addressing information, two 8-bit registers are required: 1. FSR0: composed of FSR0H:FSR0L 2. FSR1: composed of FSR1H:FSR1L 3. FSR2: composed of FSR2H:FSR2L In addition, there are registers INDF0, INDF1 and INDF2, which are not physically implemented. Reading or writing to these registers activates indirect address- ing, with the value in the corresponding FSR register being the address of the data. If an instruction writes a value to INDF0, the value will be written to the address pointed to by FSR0H:FSR0L. A read from INDF1 reads the data from the address pointed to by FSR1H:FSR1L. INDFn can be used in code anywhere an operand can be used. If INDF0, INDF1 or INDF2 are read indirectly via an FSR, all ‘0’s are read (zero bit is set). Similarly, if INDF0, INDF1 or INDF2 are written to indirectly, the operation will be equivalent to a NOP instruction and the Status bits are not affected. 5.12.1 INDIRECT ADDRESSING OPERATION Each FSR register has an INDF register associated with it, plus four additional register addresses. Performing an operation using one of these five registers determines how the FSR will be modified during indirect addressing. When data access is performed using one of the five INDFn locations, the address selected will configure the FSRn register to: • Do nothing to FSRn after an indirect access (no change) – INDFn • Auto-decrement FSRn after an indirect access (post-decrement) – POSTDECn • Auto-increment FSRn after an indirect access (post-increment) – POSTINCn • Auto-increment FSRn before an indirect access (pre-increment) – PREINCn • Use the value in the WREG register as an offset to FSRn. Do not modify the value of the WREG or the FSRn register after an indirect access (no change) – PLUSWn When using the auto-increment or auto-decrement features, the effect on the FSR is not reflected in the Status register. For example, if the indirect address causes the FSR to equal ‘0’, the Z bit will not be set. Auto-incrementing or auto-decrementing an FSR affects all 12 bits. That is, when FSRnL overflows from an increment, FSRnH will be incremented automatically. Adding these features allows the FSRn to be used as a stack pointer, in addition to its uses for table operations in data memory. Each FSR has an address associated with it that performs an indexed indirect access. When a data access to this INDFn location (PLUSWn) occurs, the FSRn is configured to add the signed value in the WREG register and the value in FSR to form the address before an indirect access. The FSR value is not changed. The WREG offset range is -128 to +127. If an FSR register contains a value that points to one of the INDFn, an indirect read will read 00h (zero bit is set), while an indirect write will be equivalent to a NOP (Status bits are not affected). If an indirect addressing write is performed when the tar- get address is an FSRnH or FSRnL register, the data is written to the FSR register, but no pre- or post-increment/ decrement is performed. LFSR FSR0,0x100 ; NEXT CLRF POSTINC0 ; Clear INDF ; register then ; inc pointer BTFSS FSR0H, 1 ; All done with ; Bank1? GOTO NEXT ; NO, clear next CONTINUE ; YES, continue |
