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Chapter 6

Chapter 6. Program Control Instruction. Introduction. program control instruction : direct the flow of a program, allow the flow to change jumps, calls, returns, interrupts, machine control instructions change in flow : CMP, TEST followed by conditional jump

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Chapter 6

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  1. Chapter 6 Program Control Instruction

  2. Introduction • program control instruction : • direct the flow of a program, allow the flow to change • jumps, calls, returns, interrupts, machine control instructions • change in flow : • CMP, TEST followed by conditional jump • relational assembly language statements : • .IF, .ELSE, .ELSEIF, .WHILE, .ENDW, .REPEAT, .UNTIL • MASM, TASM Ver.6X ~ • allow to develop control flow portions of program with C/C++ language efficiency Ch.6 Program Control Instructions

  3. 6-1 The Jump Group • JMP(jump) : allow to skip sections of a program and blanch to any part of memory for next instruction • unconditional jump, conditional jump • three type unconditional jump : Fig. 6-1

  4. Unconditional Jump(JMP) • intrasegment jump : short, near jump • Short jump(2-byte): 1 byte disp.(within +127~-128 byte) • Near jump(3-byte) : 2 byte disp.(within 32K bytes or anywhere in current code segment) • segments : cyclic in nature • intersegment, far jump(5-byte) : • any memory location within the real memory system • 80386~ (in protected mode) • Near(5-byte) : 4 byte displacement(within 2G bytes) • Far(7-byte) : 4 byte(EIP), 2 byte(CS) Ch.6 Program Control Instructions

  5. Short Jump • short jump : relative jump • distance or displacement : follow the opcode • one-byte signed number(+127~-128) : • sign-extended and added to IP/EIP • to generate the jump address within current code segment • EX. 6-1 : • label : symbolic name for memory address • SHORT directive : force a short jump • most assembler : choose best form of jump instruction • JMP START : assemble as a short jump Ch.6 Program Control Instructions

  6. Short Jump • 1st jump : 0020H – 0009H = 0017(disp. = 17H) • 2nd jump : 0002H – 0024H = FFDEH(disp. = DEH) Ch.6 Program Control Instructions

  7. Fig. 6-2 • Fig. 6-2 Ch.6 Program Control Instructions

  8. Near, Far Jump • near jump : relocatable because relative jump • signed displacement : added to IP/EIP to generate the jump address • 2 byte : 32K bytes in current code segment • 4-byte(386~ in protected mode) : 2G bytes • far jump : 5(7, 80386~) byte instruction • new offset address(IP/EIP) : byte 2,3(2~5) • new segment address(CS) : byte 4,5(6,7) • 80286~ in protected mode : CS access a descriptor that contain base address of far jump segment Ch.6 Program Control Instructions

  9. Fig. 6-3 • Fig. 6-3 Ch.6 Program Control Instructions

  10. EX. 6-2 : Near Jump • E9 0200 R JMP NEXT : only list file • R : denote a relocatable jump address of 0200H • actual machine code : E9 F6 01 • 0200H - 000AH = 01F6H Ch.6 Program Control Instructions

  11. Fig. 6-4 • Fig. 6-4 Ch.6 Program Control Instructions

  12. EX. 6-3 • far jump : FAR PTR directive, far label • far label : external to current code segment • EXTRN UP:FAR directive • a global label as a double colon(LABEL::) • ----E : external. filled in by linker when links program files

  13. Indirect Jump • jump with 16-, 32-bit reg. operand : indirect jump • contents of reg. : transferred directly into IP/EIP • JMP AX : IP ← AX, JMP EAX : EIP ← EAX • EX. 6-4 : how JMP AX access jump table • read a key, converted ASCII to binary, doubled • jump table : 16-bit offset address • Indirect Jumps using Index : double-indirect jump • [ ] form of addressing to directly access jump table • near jump JMP TABLE[SI] : IP ← [SI+TABLE] • far jump JMP FAR PTR [SI], JMP TABLE [SI] with TABLE data defined DD directive Ch.6 Program Control Instructions

  14. EX. 6-4 • EX. 6-4

  15. EX. 6-5 • EX. 6-5

  16. Conditional Jumps • conditional jump : short jump • ~ 80286(short jump) : +127 ~ -128 • 80386 ~(short, near jump) : 1, 4 bytes • test one flag bit or some more : S, Z, C, P, O • if condition under test is true : branch to the label • if condition is false : next sequential instruction • relative magnitude comparisons : • require more complicated conditional jump instructions that test more than one flag bit • Table 6-1 : conditional jump instructions Ch.6 Program Control Instructions

  17. Table 6-1 • Table 6-1

  18. Fig. 6-5 • Fig. 6-5 : order of signed, unsigned 8-bit no.s Ch.6 Program Control Instructions

  19. Conditional Jumps • unsigned : FFH is above 00H, above, below, equal • signed : FFH less than 00H, greater, less, zero • alternate form : • JE = JZ • JA(if above) = JNBE(if not below or equal) • JCXZ(jump if CX = 0), JECXZ(jump if ECX=0) • if CX/ECX = 0 : jump occur • if CX/ECX <> 0 : no jump occur • EX. 6-6 : search table for 0AH using SANSB, JCXZ Ch.6 Program Control Instructions

  20. EX. 6-6 • EX. 6-6 Ch.6 Program Control Instructions

  21. Conditional Set Instructions • conditional set instructions : • 80386~ • set a byte to either a 01H or clear a byte to 00H • useful where a condition must be tested at a point much later in the program • SETNC MEM : • places a 01H into memory location MEM if carry is cleared and • a 00H into MEM if carry is set • Table 6-2 : Ch.6 Program Control Instructions

  22. Table 6-2 • Table 6-2

  23. LOOP, Conditional LOOP • LOOP : combination of decrement CX and JNZ • ~ 80286 : DEC CX ; if CX <> 0, jump to label if CX = 0, execute next sequential instruction • 80386 ~ : CX/ECX depending on instruction mode • LOOPE(loop while equal, LOOPZ) : • jump if CX <> 0 while equal condition exist • exit the loop if CX = 0 or condition is not equal • LOOPNE(loop while not equal, LOOPNZ) : • jump if CX <> 0 while not-equal condition exist • exit the loop if CX = 0 or condition is equal • LOOPEW/LOOPED,LOOPNEW/LOOPNED:override mode Ch.6 Program Control Instructions

  24. EX. 6-7 • EX. 6-7 :

  25. 6-2 Controlling the Flow of an Assembly Language Program • relational statements • .IF, .ELSE, .ELSEIF, ENDIF, .REPEAT-.UNTIL, .WHILE-.ENDW : • easier to control the flow than conditional jump • EX. 6-8 : testing system for version of DOS • DOS INT 21H, function no. 30H : read DOS ver. • (a) : source program, (b) fully expended assembled • * : assembler-generated and -inserted statements • && : logical AND • Table 6-3 : relational operator Ch.6 Program Control Instructions

  26. Table 6-3 • Table 6-3 Ch.6 Program Control Instructions

  27. EX. 6-10 • EX. 6-10 : read a key, convert to hexadecimal • `a`(61H), `A`(41H) : 61H(41H)-57H(37H)=0AH

  28. DO-WHILE Loops • .WHILE statement : used with a condition to begin the loop • EX. 6-11 : read a key, store into array called BUF until enter key(0DH) is typed • DOS 21H, fn no. 09H Ch.6 Program Control Instructions

  29. EX. 6-11 • EX. 6-11

  30. REPEAT-UNTIL Loops • .REPEAT : defined start of loop • .UNTIL : defined end of loop, contained condition • EX. 6-14 : EX. 6-11,12 Ch.6 Program Control Instructions

  31. EX. 6-14 • EX. 6-14

  32. 6-3 Procedures • Procedure : • a group of instructions that usually performs one task • a reusable section of the software that is stored in memory once, but used as often as necessary • advantage : • save memory space • make it easier to develop software • disadvantage : • take the computer a small amount of time to link to procedure and return from it • CALL/RET : link to/return from the procedure Ch.6 Program Control Instructions

  33. Procedure • CALL : push the address of instruction following CALL(return address) on stack • RET : remove an address from stack so the program return to instruction following CALL • specific rules for storing procedure • begin with PROC, end with ENDP directive • each directive : appear with name of procedure • PROC : followed by type of procedure : NEAR,FAR • type :can be followed by the USES statement • USES statement : allow any no. of reg. to be automatically pushed and popped within procedure Ch.6 Program Control Instructions

  34. EX. 6-16 • EX. 6-16 Ch.6 Program Control Instructions

  35. CALL • near return(C3H) : remove 16-bit no. from stack, place it into IP to return from procedure in current segment • far return(CBH) : remove 32-bit no. from stack, place it into both IP, CS to return from procedure to any memory location • far procedure : global, used by all software • near procedure : local, used by a given task • CALL : differ from jump instruction • because a CALL save a return address on stack Ch.6 Program Control Instructions

  36. Near CALL • near CALL : 3(5, 80386~ in protected mode)-byte instruction • 1st byte : opcode • 2nd, 3rd byte : displacement(distance) of 32K • 2nd~5th byte : 32-bit displacement of 2G bytes • near CALL execute : • push offset address of next instruction(IP/EIP) on stack • add displacement from byte 2,3(2~5) to IP/EIP to transfer control to the procedure • CALLN(near CALL) • Fig. 6-6 : Ch.6 Program Control Instructions

  37. Fig. 6-6 • Fig. 6-6 Ch.6 Program Control Instructions

  38. Far CALL • far CALL : 5(7, 80386~ in protected mode)-byte instruction • 1st byte : opcode • 2nd 3rd byte : new IP, 4th 5th byte : new CS • 2nd~5th byte : new EIP, 6th 7th byte : new CS • far CALL execute : • push IP/EIP, CS on stack • place byte 2,3(2~5) to IP/EIP and byte 4,5(6,7) to CS to call a procedure located anywhere in memory system • CALLF(far CALL) • Fig. 6-7 Ch.6 Program Control Instructions

  39. Fig. 6-7 • Fig. 6-7 Ch.6 Program Control Instructions

  40. CALLs with Register, indirect address • CALL with register operand : • like jump, also contain a register operand • CALL BX : push IP, jump to offset address located in BX(IP ← BX) in current code segment • CALL with indirect memory address : • useful whenever different subroutines need to be chosen • CALL : also reference far pointers • CALL FAR PTR [SI] or CALL TABLE[SI] • data in table : defined as doubleword data with DD • retrieve a 32-bit address from data segment addressed by SI, use it as address of a far procedure Ch.6 Program Control Instructions

  41. EX. 6-17 • EX. 6-17 : display ‘OK’

  42. RET • RET : real mode(80386~ in protected mode) • near RET: remove 16-bit(32-bit), place it into IP/EIT • far : remove 32-bit(6 bytes), place it into IP/EIP, CS • near, far return : defined in procedure’s PROC • other form : RET n • n(bytes) : add n to contents of SP after return address is removed from stack • push passing parameters on stack before calling procedure • if these parameters are to be discarded upon return, RET contains a no. that represents the no. of bytes pushed to stack as parameters Ch.6 Program Control Instructions

  43. Fig. 6-8 : near return • Fig 6-8 Ch.6 Program Control Instructions

  44. EX. 6-19 • EX. 6-19 Ch.6 Program Control Instructions

  45. RET • RETN : CALLN • RETF : CALLF • passing parameters to a procedure : • 1. to use one of the CPU register : MOV CX, TI • 2. to use a memory location : MOV TEMP, TI • 3. to pass the address of memory location : MOV SI, OFFSET TI • 4. to pass the parameters on the stack : (EX. 6-19) MOV DX, TI, PUSH DX • 5. to use stack frame : ENTER, LEAVE(p.211) Ch.6 Program Control Instructions

  46. 6-4 Introduction to Interrupt • 1. hardware-generated CALL : external interrupt • externally derived from a hardware signal • 2. software-generated CALL : internal, exception • internally derived from the execution of an instruction or by some other internal event() • interrupt : interrupts the program by calling an interrupt service procedure or interrupt handler • interrupt vector : • in real : 4-byte no. stored in 1st 1024 bytes(~0003FFH) 256(00H~FFH)  4byte = 1024byte • protected : replaced by interrupt descriptor table Ch.6 Program Control Instructions

  47. Interrupt Vectors • Table 6-4 : 256 different interrupt vectors in real • each contain address of an interrupt service procedure for IP, CS • Intel reserve the 1st 32 interrupt vector(~1FH) : for present , future µ • remaining : available for user • some of reserved : for error that occur during execution of software, such as divide error interrupt • some of reserved : for coprocessor • others : occur for normal events in the system • vectors 1-6,7,9,17 : function in real, protected mode • remaining : only in protected mode Ch.6 Program Control Instructions

  48. Table 6-4 • Table 6-4 Ch.6 Program Control Instructions

  49. Interrupt Instructions • software interrupt instruction : special type of CALL • INT, INTO, INT3 • each of these instruction : • 1. in real, fetches vector from interrupt vector table • 1. in protected, fetches an interrupt descriptor from interrupt descriptor table • 2. calls the interrupt service procedure • interrupt call : similar to far CALL instruction • because placed return address(IP/EIP, CS) on stack • different : pushed flags, then pushed return address • fetched new value IP/EIP, CS from vector Ch.6 Program Control Instructions

  50. INTs • INT n : 256 different software interrupt instruction • type no. n : 0 ~ 255(00H ~ FFH) • INT 100 : uses interrupt vector no. 100(64H) • memory address in IVT : 190H~193H • 0110 0100(64H) → shift left 2 → 01 1001 0000(190H) • address of interrupt vector in real : • multiplying type no. times 4(each vector : 4 bytes) • address of interrupt descriptor in protected : • multiplying type no. times 8(each descriptor : 8 byte) • INT : 2-byte long(1st:opcode, 2nd:vector type no.) • INT 3: 1-byte special software interrupt for breakpoints Ch.6 Program Control Instructions

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