16.317 Microprocessor Systems Design I Instructor: Dr. Michael Geiger Fall 2013 Lecture 30: PIC programming Lecture outline Announcements/reminders Review: complex operations HW 6 due 11/25 No lecture 11/27 Working with multiple registers Conditional jumps Shift/rotate Today’s lecture: multi-byte data 1/11/2017 Microprocessors I: Lecture 29 2 Review: complex operations Multiple registers Data must be transferred through working register Conditional jumps Usually btfsc/btfss instruction + goto Equality/inequality—use subtract in place of CMP If you subtract X – Y: X > Y Z = 0, C = 1 X == Y Z = 1, C = 1 X < Y Z = 0, C = 0 X <= Y Z == C X != Y Z = 0 X >= Y C = 1 Shift/rotate 1/11/2017 Manipulate carry before operation Use loop for multi-bit shift/rotate Microprocessors I: Lecture 10 3 Examples Translate these x86 operations to PIC code Assume that there are registers defined for each x86 register (e.g. AL, AH, BL, BH, etc.) OR SUB JNZ JL SAR ROL 1/11/2017 AL, BL BL, AL label label AL, 1 AL, 5 Microprocessors I: Lecture 29 4 Example solution OR AL, BL movf BL, W iorwf AL, F SUB BL, AL movf AL, W subwf BL, F JNZ ; W = BL ; AL = AL OR W = AL OR BL ; W = AL ; BL = BL – W = BL – AL label btfss STATUS, Z ; Skip goto if Z == 1 (if goto label ; previous result == 0) 1/11/2017 Microprocessors I: Lecture 29 5 Example solution (continued) JL label btfsc goto btfss goto End: STATUS, Z End STATUS, C label 1/11/2017 ; If Z == 0, check C ; Otherwise, no jump ; If C == 1, no jump ; Jump to label ; End of jump Microprocessors I: Lecture 29 6 Example solution (continued) SAR AL, 1 bcf STATUS, C btfsc AL, 7 bsf STATUS, C rrf 1/11/2017 AL, F ;C=0 ; Skip if MSB == 0 ; C = 1 if MSB == 1 ; C will hold copy of ; MSB (keeping sign ; intact) ; Rotate right by 1 Microprocessors I: Lecture 29 7 Example solution (continued) ROL L: AL, 5 movlw 5 movwf COUNT bcf STATUS, C btfsc AL, 7 bsf STATUS, C rlf AL, F decfsz COUNT goto 1/11/2017 ;W=5 ; COUNT = W = 5 ;C=0 ; Skip if MSB == 0 ; C = 1 if MSB == 1 ; C will hold copy of ; MSB (bit rotated into ; LSB) ; Rotate left by 1 ; If COUNT == 0, don’t ; restart loop L Microprocessors I: Lecture 29 8 Multi-byte data Logical operations can be done byte-by-byte Arithmetic and shift/rotate operations require you to account for data flow between bytes Carry/borrow in arithmetic Bit shifted between bytes in shift/rotate Order of these operations is important 1/11/2017 Arithmetic: must do least significant bytes first Shift/rotate: move through bytes in same order as shift bits being shifted will move through carry Microprocessors I: Lecture 9 9 Working with 16-bit data Assume a 16-bit counter, the upper byte of the counter is called COUNTH and the lower byte is called COUNTL. Decrement a 16-bit counter movf COUNTL, F btfsc STATUS, Z decf COUNTH, F decf COUNTL, F Test a 16-bit variable for zero movf COUNTL, F btfsc STATUS, Z movf COUNTH, F btfsc STATUS, Z goto BothZero CarryOn 1/11/2017 ; Set Z if lower byte == 0 ; if so, decrement COUNTH ; in either case decrement COUNTL ; Set Z if lower byte == 0 ; If not, then done testing ; Set Z if upper byte == 0 ; if not, then done ; branch if 16-bit variable == 0 Microprocessors I: Lecture 9 10 Examples Translate these x86 operations to PIC code Assume that there are registers defined for each x86 register (e.g. AL, AH, BL, BH, etc.) 16-bit values (e.g., AX) must be dealt with as individual bytes MOVZX MOVSX INC SUB RCL 1/11/2017 AX, BL AX, BL AX BX, AX AX, 5 Microprocessors I: Lecture 9 11 Example solutions MOVZX AX, BL movf BL, W movwf AL clrf AH MOVSX AX, BL movf BL, W movwf AL clrf AH btfsc AL, 7 decf AH, F 1/11/2017 ; Copy BL to W ; Copy W to AL ; Clear upper byte ; Copy BL to W ; Copy W to AL ; Clear upper byte ; Test sign bit ; If sign bit = 1, set ; AH = 00 - 1 = 0xFF Microprocessors I: Lecture 9 12 Example solutions INC AX incf btfsc incf AL, F STATUS, Z AH, F SUB movf subwf btfss decf BX, AX AL, W BL, F STATUS, C BH, F movf AH, W subwf BH, F 1/11/2017 ; Increment low byte ; Check zero bit ; If Z == 1, increment ; high byte ; Copy AL to W ; BL = BL – AL ; Check carry ; If C == 0 (borrow is 1), ; decrement BH before sub. ; Copy AH to W ; BH = BH - AH Microprocessors I: Lecture 9 13 Example solutions RCL AX, 5 movlw 5 movwf COUNT L: rlf AL, F rlf AH, F decfsz COUNT, F goto L 1/11/2017 ;W=5 ; COUNT = W = 5 ; Assumes register ; COUNT is defined ; Rotate low byte ; Bit transferred from ; low to high byte is ; now in carry ; Rotate high byte ; Decrement & test COUNT ; Return to start of loop if ; COUNT != 0 Microprocessors I: Lecture 9 14 Final notes Next time: Continue with complex operations—multi-byte data Reminders: 1/11/2017 HW 6 to be posted; due date TBD Microprocessors I: Lecture 29 15
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