Файл: The quintessential PIC microcontroller (S. Katzen, 2000).pdf

6. Subroutines and Modules 163

root. An example giving √65 = 8 is given in Fig. 6.9(a) using this series approach. A flowchart visualizing the task list is also given in Fig. 6.9(b).

Program 6.11 Coding the square root subroutine.

;************************************************************

;* FUNCTION: Calculates the square root of a 16-bit integer *

;************************************************************

;Local declarations

;Task 3: DO

;Task 3(a): Number - I

; Task 4: Return loop count as the square root

SQR_END movf COUNT,w ; Copy into W return

164 The Quintessential PIC Microcontroller

The coding in Program 6.11 follows the task list closely. The maximum value of the loop count is FFh, as √65535 = 255. Thus a single byte at File 35h is reserved for this local variable. Similarly the maximum possible value of the magic number is 511 (1FFh) and so the two registers File 36:7h are reserved for this local variable. This of course means that Task 3(a) entails a double-byte subtraction. The coding is somewhat simplified as the high byte of I, that is I_H, is never more than 01h and so a borrow from the lower byte can be added to a copy of I_H before the high-byte subtract to return the borrow without overflow. If a borrow is generated from this high-byte subtraction the outcome is under zero and the loop is exited. Otherwise COUNT is incremented and I augmented by two. Actually the latter is always twice COUNT plus one, so COUNT is not needed. Instead, on return the 16-bit value I can be shifted once right. This divides by 2 and by throwing away the one that pops out into the carry, e ectively subtracts by one – I is always odd and so its least significant bit is always 1. Try coding this alternative arrangement.

Example 6.6

Repeat Example 5.5, which multiplies a byte by ten, but using a software stack for data storage and parameter passing. You may assume that the multiplicand byte is in memory at File 46h.

Solution

The global declarations for the subroutine of Program 6.12 and calling procedure is:

;..........................and so on

;Get ready to call up the X10 subroutine

movf PSP,w ; 1st point to current stack position

call X10 ; Now call subroutine

;On return PSP is returned to original position

;and product is at PSP+3:PSP+2

NEXT_MAIN ..... ... ; Continuation of main routine

6. Subroutines and Modules 165

Program 6.12 uses the same technique as the original routine. First the multiplicand is shifted left once to multiply by two and then two further shifts multiplies by eight. The two resulting 16-bit data are then added to give the product. In the same manner as Program 6.6, the File Select Register is moved up and down to point the the appropriate datum as the

Program 6.12 Using a software stack to pass parameters and to provide a workspace. (continued next page).

;************************************************************

;* FUNCTION: Xs byte XCAND by 10 giving double-byte product *

; ************************************************************

166 The Quintessential PIC Microcontroller

Program 6.12 (continued.) Using a software stack to pass parameters and to provide a workspace.

return

; ************************************************************

program progresses. The double-byte product can be accessed relative to the Pseudo Stack Pointer by the caller. Unlike Program 6.6 this PSP is not altered when pushing out the multiplicand nor in the subroutine. This is because the subroutine is a dead end in that it can never call another subroutine. Thus a new stack frame need not be formed.

Example 6.7

In order to ensure that the 7-segment decoder subroutine of Program 6.4 does not cause the PCL register to overflow when the o set is added, a programmer has used the directive org (ORiGin – see page 200) to tell the assembler to locate the subroutine at the absolute instruction address 700h – as shown in Program 6.13. When the subroutine is tested by calling from another part of the program in the store somewhere lower than 700h the system fails and performs unpredictably. What has gone wrong?

Program 6.13 The software 7-segment decoder revisited.