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PIC18LF26K22 Datasheet(PDF) 71 Page - Microchip Technology |
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PIC18LF26K22 Datasheet(HTML) 71 Page - Microchip Technology |
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71 / 492 page  2010 Microchip Technology Inc. Preliminary DS41412B-page 71 PIC18(L)F2X/4XK22 A CALL type instruction causes a push onto the stack; the Stack Pointer is first incremented and the location pointed to by the Stack Pointer is written with the contents of the PC (already pointing to the instruction following the CALL). A RETURN type instruction causes a pop from the stack; the contents of the location pointed to by the STKPTR are transferred to the PC and then the Stack Pointer is decremented. The Stack Pointer is initialized to ‘00000’ after all Resets. There is no RAM associated with the location corresponding to a Stack Pointer value of ‘00000’; this is only a Reset value. Status bits indicate if the stack is full or has overflowed or has underflowed. 5.1.2.1 Top-of-Stack Access Only the top of the return address stack (TOS) is readable and writable. A set of three registers, TOSU:TOSH:TOSL, hold the contents of the stack location pointed to by the STKPTR register (Figure 5-2). This allows users to implement a software stack if necessary. After a CALL, RCALL or interrupt, the software can read the pushed value by reading the TOSU:TOSH:TOSL registers. These values can be placed on a user defined software stack. At return time, the software can return these values to TOSU:TOSH:TOSL and do a return. The user must disable the Global Interrupt Enable (GIE) bits while accessing the stack to prevent inadvertent stack corruption. FIGURE 5-2: RETURN ADDRESS STACK AND ASSOCIATED REGISTERS 5.1.2.2 Return Stack Pointer (STKPTR) The STKPTR register (Register 5-1) contains the Stack Pointer value, the STKFUL (stack full) Status bit and the STKUNF (Stack Underflow) Status bits. The value of the Stack Pointer can be 0 through 31. The Stack Pointer increments before values are pushed onto the stack and decrements after values are popped off the stack. On Reset, the Stack Pointer value will be zero. The user may read and write the Stack Pointer value. This feature can be used by a Real-Time Operating System (RTOS) for return stack maintenance. After the PC is pushed onto the stack 31 times (without popping any values off the stack), the STKFUL bit is set. The STKFUL bit is cleared by software or by a POR. The action that takes place when the stack becomes full depends on the state of the STVREN (Stack Over- flow Reset Enable) Configuration bit. (Refer to Section 24.1 “Configuration Bits” for a description of the device Configuration bits.) If STVREN is set (default), the 31st push will push the (PC + 2) value onto the stack, set the STKFUL bit and reset the device. The STKFUL bit will remain set and the Stack Pointer will be set to zero. If STVREN is cleared, the STKFUL bit will be set on the 31st push and the Stack Pointer will increment to 31. Any additional pushes will not overwrite the 31st push and STKPTR will remain at 31. When the stack has been popped enough times to unload the stack, the next pop will return a value of zero to the PC and sets the STKUNF bit, while the Stack Pointer remains at zero. The STKUNF bit will remain set until cleared by software or until a POR occurs. 00011 001A34h 11111 11110 11101 00010 00001 00000 00010 Return Address Stack <20:0> Top-of-Stack 000D58h TOSL TOSH TOSU 34h 1Ah 00h STKPTR<4:0> Top-of-Stack Registers Stack Pointer Note: Returning a value of zero to the PC on an underflow has the effect of vectoring the program to the Reset vector, where the stack conditions can be verified and appropriate actions can be taken. This is not the same as a Reset, as the contents of the SFRs are not affected. |
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