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ELE22MIC Lecture 7

This lecture continues from Lecture 5 and covers the topics of selecting conditional jumps, coding for-loops in AS11, and increasing the size of arithmetic add, mul, and interrupt processing. Examples are provided for for-loops and repeat loops.

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ELE22MIC Lecture 7

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  1. ELE22MIC Lecture 7 • Instruction Set Part 5-continued from Lecture 5 • Selecting Conditional Jumps • Coding For-Loops (in AS11) • Increasing the size of arithmetic Add, Mul • Interrupt processing

  2. For-Loop Example org <RAM> ; say $2000 for HC-COM Count RMB 1 ; Uses Count, Acc A and Flags org <CODE> ; say $8000 or $2100 for HC-COM FiveSomethings: ; for(Count = 0; Count <> 5; Count++); CLR Count ; Count = 0; LoopAgain: LDAA Count ; A = Count CMPA #5 ; (A == 5)? BEQ ExitLoop ; Yes, exit loop JSR DoSomething ; { DoSomething (); } ; do it 5 times INC Count ; Count++; JMP LoopAgain: ; ExitLoop: RTS

  3. Another For-Loop Example org <CODE> ; Uses: AccB = LoopCounter, AccA = LoopLimit, CCR ASomethings: ; for (B = 0; B <> A; B++); CLRB ; B = 0; LoopAgain: ; CBA ; (A == B)? BEQ ExitLoop ; Yes, exit loop JSR DoSomething ; { DoSomething (); } do it A times INCB ; B++; JMP LoopAgain: ; ExitLoop: RTS

  4. Repeat .. DownTo 0 Example org <CODE> ; Uses: = N, CCR NSomethings: ; RepeatAgain: ; repeat { JSR DoSomething ; { DoSomething () // do it A times DEC N ; N--; ; } BNE RepeatAgain ; until (N == 0) ExitLoop: RTS org <RAM> N RMB 1

  5. More Robust Repeat Example org <CODE> ; Uses: = I, CCR NSomethings: ; RepeatAgain: ; repeat { JSR DoSomething ; { DoSomething () // do it A times DEC I ; I--; // Alters CCR bits: N, Z, V ; } BGT RepeatAgain ; until (I <= 0), I.e. if !(I>0) Repeat ExitLoop: RTS org <RAM> I RMB 1 ; N can be in range 0..127

  6. Arithmetic/Logical Shift & Rotate ROtate Left AccA ROtate Right AccA Arithmetic Shift Left AccA Arithmetic Shift Right AccA Note Negative Number Sign Extended Arithmetic Shift Right AccA Note Positive Number Sign Extended Logical Shift Right AccA

  7. 8-bit x 8-bit Multiply Example (1) org <RAM> OP1 RMB 2 ; RMB Allocate 2 Bytes =16 bits OP2 RMB 1 COUNT RMB 0 org <CODE> Multiply: ;perform 8-bit x 8-bit multiply -> 16bit result ; D = OP1 * OP2 CLR OP1 LDAA #8 ; AccA = 8 STAA Count ; Count = AccA = 8 LDAA #0 ; Result is Accumulated in A:B = D LDAB #0 ; initialised to 0 -> Continued next Page

  8. 8-bit x 8-bit Multiply Example (2) Loop: LSR op2+1 ; Carry Flag now = LSB of OP2 BCC DontAdd ; Carry = 0, so don’t accumululate ADDD op1 ; Add (OP1 << ?) to 16-bit result DontAdd: LSL op1+1 ; Shift Low Byte, MSB of OP1 -> CF ROL op1 ; Rotate High Byte, CF-> LSB of OP1+1 DEC Count ; from 8 downto 1 BNE Loop ; If (Count==0) loop is done. RTS ; returns result in AccD = OP1 * OP2

  9. Adder/Subtracter Circuit Adder/Subtracter from Digital Systems: Principles and Applications by Tocci & Widmer. PHIPE.

  10. 32-bit Add Unsigned Example op1 RMB 2 ; Allocate 2 bytes = 16 bits for operand 1 op2 RMB 2 ; Allocate 2 bytes = 16 bits for operand 2 result RMB 4 ; Allocate 4 bytes = 32 bits for result Add32bit: LDD op1+2 ; Get source operand (low word) ADDD op2+2 ; 16 bit add, CarryFlag is set STD result+2 ; Mem[Result] = D = low word result LDD op1 ; Get source operand (high word) ADCB op2+1 ; 8 bit add with carry (Low Byte) ADCA op2 ; 8 bit add with carry (High Byte) STD result ; memory[result] = D = A:B ; result + 2 = High word result. CarryFlag is set if Carry Out

  11. 16-bit x 16-bit Multiply Example (1) Modify the previous 8-bit x 8-bit example: org <RAM> OP1 RMB 4 ; Allocate 4 Bytes = 32 bits OP2 RMB 2 ; Allocate 2 Bytes =16 bits RESULT RMB 4 ; Allocate 4 bytes = 32 bits for result COUNT RMB 1 org <CODE> To Shift Right all 16 bits: LSR OP2 ; LS Bit of MS Byte -> CarryFlag ROR OP2+1 ; CarryFlag -> MS Bit of LS Byte ; LSB of OP2 -> CarryFlag- Used forBCC…

  12. 16-bit x 16-bit Multiply Example (2) ADDD OP1 needs to be 32 bit add to result To Shift Left all 32 bits: (Multiply by 2) LSL OP1+3 ; Shift Lowest Byte, MSB of OP1+3 -> CF ROL OP1+2 ; Next, CF-> LS Bit of OP1+2, MS Bit->CF ROL OP1+1 ; and again ROL OP1 ; Finally Rotate CF -> MSBit of High Byte 68HC11 represents numbers stored in Memory in “Big Endian” format: i.e. Most significant byte is at lowest address, next most at address+1... Intel 80x86 is “Little Endian” : LS Byte -> Lowest Memory Address. ASCII Strings Always start at lowest # address in Big & Little

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