1. Microprocessor Programming II To discuss more complicated programming techniques Flag control instructions Compare and jump Subroutines Loop and string instructions

2. Flag instructions Can affect the setting of flags directly Can store value of AH into flags Can clear/set the carry Can clear/set interrupt Complement carry flag (0->1, 1->0)

3. Flag Instructions MnemonicMeaningFlags Affected LAHFLoad AH from flagNone SAHFStore AH into flagsSF, ZF, AF, PF, CF CLCClear carry CF  STCSet carry CF  CMCComplement carry CF  CLIClear interruptIF STISet interruptIF

4. Flag instruction Can you identify one application for clearing the carry flag? Example before a rotation through carry Example if you want to calculate the average value of a series of data, the first time you use ADC

5. Compare instruction To compare two 8-bit or 16-bit numbers Operands can stored in Register, memory, ACC (AX), or an immediate The compare operation will affect the following flags: overflow, sign, zero, auxiliary carry, parity, and carry Instruction mnemonic is CMP Usually use compare operation when decision to branch (or jump) is required

6. Compare In the compare operation, the source operand is subtracted from the destination operand But the result of the subtraction is not saved (i.e. the values in the source and destination did not change) Flags are changed according to the subtraction result After the compare, we can do a conditional jump based on the flags status

7. Compare In C++, we do if(x==0) –cmp x, #00h (compare X and 0 so if x = 0 then x-0 will set the zero flag!!) –JZ abc ; JZ – jump if zero flag is set

8. Exercise Want to do if (AX > 10) After cmp AX, #10 then which flag should be checked?

9. Example CMP 10011001, 00011011 Do 10011001 - 00011011 Results of flags after the compare AF is set CF is clear OF is set PF is set SF is clear ZF is clear

10. Exercise Describe what will happen to the status flags (C, S, O, Z) as the sequence of instructions that follow is executed MOVAX, 1234H MOV BX, ABCDH; this is a 16-bit value CMP AX, BX Assume that initially all flags are reset (0) The CF=1

11. Change the flow of the program Normal program flow Is top-down The flow of a program can also be changed by: jump Conditional Jump Subroutine call looping jmp abc abc:

12. Program flow control Jump (jmp) – after the jump, the program will execute instructions at the jump label and will not return Syntax: jmp label Subroutine call (call) – program will go to the location of the subroutine, execute the subroutine and when it finishes return back to the original program Syntax: call function (or acall function) –Function is a user defined name representing the subroutine

13. JUMP instruction To alter the execution path of instructions in the program By changing the values of the code segment register and the instruction pointer Program execution is not intended to return to the next sequential instruction after the jump, so no return linkage is saved Two kinds of jump operation: unconditional and conditional

14. Unconditional Jump As its name implies, there are no status (condition) requirements imposed for the jump to occur As the instruction is executed, the jump always takes place to change the execution sequence Conditional Jump The jump operation depends on some status conditions ( for example jump if Carry is set!) If condition(s) is/are met, then the jump takes place; Otherwise execution continues with the next sequential instruction of the program

15. Unconditional jump There are 2 kinds of unconditional jump: intrasegment jump, and intersegment jump Intrasegment jump is limited to addresses within the current code segment (only the IP value is changed) Intersegment jump permits jumps from one code segment to another (both CS and IP values are changed)

16. Examples for unconditional jump JMP $ -14 Short-label – an 8-bit number is coded as an immediate operand to specify a signed displacement (+127 to –128) IP = IP - 14 JMP START Near-label – operand specifies a new value of IP with a 16-bit displacement –START is a label

17. Intersegment jump Use a 32-bit immediate operand to specify the jump address The operand can be called a far-label The first 16 bits are loaded to IP and the next 16 bits are loaded into the CS

18. Example of unconditional jump XORBX,BX; why doing this ????? Start:movAX, #1 ADDAX,BX JMPNEXT NEXT:mov BX, AX JMPstart

19. Conditions Conditions that can be referenced by a conditional jump: CF, PF and OF Mnemonic for some condition jump operations JC jump if CF=1 JS jump if SF=1 (could set by CMP, SUB, ADD, AND, OR, XOR ) JB jump if CF =1 (but CF is set after a CMP operation ) JNL jump if first operand is not less than second operand in the CMP operation (SF = OF)

20. Exercise The program that follows implements what is known as a delay loop MOVCX, 1000 DLY: DEC CX JNZDLY ; jump if not zero (zero is specified by what?) NXT: ----- How many times does the JNZ DLY get executed? Change the program so that JNZ DLY is executed just 17 times

21. Subroutine call Address (offset) instruction 1200Call sub_abc 1202Mov AX, BX 1400Sub_abc: RET Need to store the return address!!!! But in where?

22. PUSH and POP PUSH and POP are used to stored data onto the stack. Stack is used as a temporary storage When performing PUSH AX ; value of AH is stored in SP-1, value of AL is stored in SP-2, the new value of SP is SP-2 SP – stack pointer (offset) During a POP AX Content of SP is stored in AL Content of SP+1 is stored in AH New value of SP is SP+2

23. Stack operation TOS – Top Of Stack

24. Stack operation

25. Subroutines A subroutine is a special segment of program that can be called for execution from any point in a program (similar to a function in C program) To invoke a subroutine, we perform a CALL Using subroutines can reduce the size of the program and make the program easy to read When performing a CALL operation, value of IP is changed in order to branch to the location of the subroutine When the subroutine is completed then the program returns to the main program so the last statement in the subroutine must be RETURN

26. Subroutine There are intrasegment call and intersegment call During a CALL operation, value of IP is stored in the STACK The IP value saved is the instruction following the CALL operation Example: CALL 1234 (1234 is a 16-bit displacement that gets added to IP) Example: Call delay –Delay is the name of a subroutine

27. RETURN operation Mnemonic for RETURN is RET During the return operation, value of IP or both the values of IP and CS that were saved on the stack to be returned back to their corresponding registers

28. Example Write a procedure named SUM that adds 2 numbers 5 and 31 together and places their sum in DX. Assume that this procedure is to be called from another procedure in the same code segment and that at the time it is to be called, DX contains the value of ABCD 16 and this value must be saved at entry of the procedure and restored at its completion.

29. Sample solution Sum: push dx movdx, 5 adddx,31 popdx ret

30. Loop instructions A mechanism to repeat a sequence of operations repeatedly Three different operations: loop, loope/loopz, and loopne/loopnz The number of looping is stored in CX

31. Looping Whenever LOOP is executed, the contents of CX are first decremented by 1 and then checked to determine if they are equal to 0 If CX equals to 0, then return to the instruction following the loop statement

32. Example mov cx, count Next:…….. Loop Next Syntax of the loop operation Do something else Loop repeated Check CX For ADuC832, one looping function is DJNZ (do jump if not zero )

33. Loop while equal (loope) Loop while equal (loope) checks the contents of both CX and the ZF flag. Each time the loop instruction is executed, CX decrements by 1 without affecting the flags, its contents are checked for 0, and the state of ZF that results from execution of the previous instruction is tested for. Loope (loop if CX != 0 and ZF = 1) Loopne (loop if CX != 0 and ZF = 0)

34. Exercise Given the following sequence of instructions: mov AX, 0 mov DS, AX movAL, 05 movDI, A000 movCX, 0FH Again:incDI cmp[DI], AL loopneAgain Next: Explain what will happen as they are executed Ans. Search for 05 for addresses A001H to A010H

35. String instructions String refers to a series of data words (or bytes) that reside in consecutive memory locations. In C++, you get the strcpy function Can move a data from a block of memory to a block elsewhere To scan a string of data elements stored in memory looking for a specific value To compare two strings Five basic string instructions

36. String Operation String db “abcd” a d Memory (Data Segment) 0000 FFFF SI Memory (Extra Segment) 0000 FFFF DI SourceDestination

37. Basic string instructions Mnemonicmeaningformat MOVSMove stringMOVs operand MOVSBMove string byte MOVSB MovswMove string word Movsw CmpsCompare stringCmps operand ScasScan stringScas operand LodsLoad stringLods operand StosStore stringStos operand

38. String operations The instructions MOVS, MOVSB, MOVSW all perform the same basic operation. An element of the string specified by the source index (SI) register with respect to the current data segment (DS) register is moved to the location specified by the destination index (DI) register with respect to the current extra segment (ES) register It can be a byte or word operation After the move is completed, the contents of both SI and DI are automatically incremented or decremented by 1 for byte move, or by 2 for word move

39. Direction flag Direction flag selects the auto-increment or the auto-decrement operation for the DI and SI registers during string operations. The direction flag is used only with the string instructions. CLD – clear direction flag (D=0 auto increment) STD – set (D=1 auto decrement)

40. Move string Increment or decrement is controlled by the direction flag DF Example of copying N bytes of characters from blk1addr to blk2addr MovAX, datasegaddr ; load AX with data segment MovDS, AX; address MovES, AX; ES = DS point to the same segment LEASI, blk1addr ; note this is moving the address LEADI, blk2addr MovCX, N CLD; clear the direction flag - increment Next:movsb Loopnext HLT; halt = stop

41. Compare strings Compare operations perform: subtracts the destination operand from the source operand and adjusts flags CF, PF, AF, ZF, SF, and OF accordingly. The result of subtraction is not saved Operation does not affect the operands Example –cmpsb ; compare byte no operand Source is pointed to by the address in SI Destination is specified by DI SI and DI are updated after the operation and pointed to the next element

42. Scan string Scan string – look for a character (target) in a string AL, or AX stores a target element Destination string is referred by DI and ES Flags are adjusted based on the operation and DI incremented or decremented

43. Example for scan Example:mov Ax, 0 movDS, AX movES, AX movAL, 05H movDI, A000H movCX, 0F CLD; clear direction flag Again:scasb; scan byte no operand loopneagain ; stop if find the same byte Next:

44. Load and store string To move string elements between the accumulator (AX or AL) and memory LODS loads either a byte or a word from a string in memory into AL or AX, respectively Address is derived from SI and DS Example:LODSW Load AX with a word Value of SI is incremented by 2 STOS stores a byte from AL or a word from AX into a string location in memory Location is generated by ES and DI

45. Example movAX, 0 movDS, AX movES, AX movAL, 05 movDI, A000 movCX, 0F CLD Again:stosb loopneagain Next: Store the value 5 into location A000 to A00F

46. Exercise Describe what will happen as the following sequence of instructions is executed CLD MovAX, data_segment MovDS, AX MovAX, extra_segment MovES, AX MovCX, 20 LEASI, offset_master LEADI, offset_copy Ops: CMPSB Loop ops Ans. The two strings are compared for 20 bytes