1. Assemblers System Software by Leland L. Beck Chapter 2

2. Chap 2 Role of Assembler Source Program Assembler Object Code Loader Executable Code Linker

3. Chap 2 Chapter 2 -- Outline n Basic Assembler Functions n Machine-dependent Assembler Features n Machine-independent Assembler Features n Assembler Design Options n Implementation examples

4. Chap 2 Introduction to Assemblers n Fundamental functions u translating mnemonic operation codes to their machine language equivalents u assigning machine addresses to symbolic labels used by the programmer n Machine dependency u different machine instruction formats and codes

5. Chap 2 Example Program (Fig. 2.1) n Purpose u The main routine reads records from input device (code F1) u copies them to output device (code 05) u The main routine calls subroutine RDREC to read a record into the buffer and subroutine WRREC to write a record from the buffer to the o/p device

6. Chap 2 u The end-of-record marked with NULL char(00) u If (size(rec) > len(buff) ) only 1 st 4096 bytes are copied u at the end of the file, writes EOF on the output device, then RSUB to the operating system u Lines beginning with dot (.) represents Comment u Line number are just for your reference u program program

7. Chap 2

8. Figure 2.1 (Pseudo code) Program copy { save return address; cloop: call subroutine RDREC to read one record; if length(record)=0 { call subroutine WRREC to write EOF; } else { call subroutine WRREC to write one record; goto cloop; } load return address return to caller }

9. Chap 2 An Example (Figure 2.1, Cont.) Subroutine RDREC { clear A, X register to 0; rloop: read character from input device to A register if not EOR { store character into buffer[X]; X++; if X < maximum length goto rloop; } store X to length(record); return } EOR: character x‘00’

10. Chap 2 An Example (Figure 2.1, Cont.) Subroutine WDREC { clear X register to 0; wloop: get character from buffer[X] write character from X to output device X++; if X < length(record) goto wloop; return }

11. Chap 2 Assembler Directives n Pseudo-Instructions u Not translated into machine instructions u Providing information to the assembler n Basic assembler directives u START u END u BYTE u WORD u RESB u RESW

12. Chap 2

15. Assembler’s functions n Convert mnemonic operation codes to their machine language equivalents Ex Translate STL to 14 (line no 10) n Convert symbolic operands to their equivalent machine addresses Ex: RETADR TO 1033 (line no 10) n Build the machine instructions in the proper format n Convert the data constants to internal machine representations Translate EOF to 454F46 (line no 80) n Write the object program and the assembly listing

16. Chap 2 Difficulties: Forward Reference n Forward reference: reference to a label that is defined later in the program. LocLabelOperatorOperand 1000FIRSTSTLRETADR 1003CLOOPJSUBRDREC … ………… 1012JCLOOP … ………… 1033RETADRRESW1

17. Chap 2 n Because of forward reference most assemblers make two passes over the source program n In addition the assembler must process assembler directives n The assembler must write the generated object code onto some o/p device. This object code later loaded to memory for execution n The simple object program format consist of 3 types of records

18. Chap 2 Object Program Format n Header Record Col. 1H Col. 2~7Program name Col. 8~13Starting address (hex) Col. 14-19Length of object program in bytes (hex) n Text Record Col.1 T Col.2~7Starting address in this record (hex) Col. 8~9Length of object code in this record in bytes (hex) Col. 10~69Object code (69-10+1)/6=10 instructions n End Record Col.1E Col.2~7Address of first executable instruction (hex) (END program_name)

19. Chap 2 Fig. 2.3 H COPY 001000 00107A T 001000 1E 141033 482039 001036 281030 301015 482061... T 00101E 15 0C1036 482061 081044 4C0000 454F46 000003 000000 T 002039 1E 041030 001030 E0205D 30203F D8205D 281030 … T 002057 1C 101036 4C0000 F1 001000 041030 E02079 302064 … T 002073 07 382064 4C0000 05 E 001000

20. Chap 2 Two Pass Assembler n Pass 1 u Assign addresses to all statements in the program u Save the values assigned to all labels for use in Pass 2 u Perform some processing of assembler directives n Pass 2 u Assemble instructions u Generate data values defined by BYTE, WORD u Perform processing of assembler directives not done in Pass 1 u Write the object program and the assembly listing

21. Chap 2 Two Pass Assembler n Read from input line u LABEL, OPCODE, OPERAND Pass 1Pass 2 Intermediate file Object codes Source program OPTAB SYMTAB

22. Chap 2 Data Structures n Operation Code Table (OPTAB) n Symbol Table (SYMTAB) n Location Counter(LOCCTR)

23. Chap 2 OPTAB (operation code table) n Content u menmonic, machine code (instruction format, length) etc. F Used for PASS1: Used to lookup mnemonic opcode and validate opcode in source program PASS2: translate them to their machine language equivalent n Characteristic u static table n Implementation u array or hash table, easy for search

24. Chap 2 SYMTAB (symbol table) n Content u label name, value, flag, (type, length) etc. u Used for F PASS 1: Labels are entered into Symbol table as they are encountered in source program along with their assigned address (from LOCCTR) F PASS 2: Symbols used as operands are looked up in Sym tab to obtain the addresses to be inserted in the assembler instructions n Characteristic u dynamic table (insert, delete, search) n Implementation u hash table, non-random keys, hashing function COPY1000 FIRST 1000 CLOOP1003 ENDFIL1015 EOF1024 THREE102D ZERO1030 RETADR1033 LENGTH1036 BUFFER1039 RDREC2039

25. Chap 2 Location Counter(LOCCTR) n A variable used to help in assignment of addresses n LOCCTR is initialized to the beginning address specified in the START statement n After each source statement is processed the length of the assembled instruction or data area o be generated is added to LOCCTR n Whenever we reach a label in the source program, the current value of LOCCTR gives the address to be associated with the label

26. Chap 2

28. Example program SUMSTART4000 FIRSTLDXZERO LDAZERO LOOPADDTABLE,X TIXCOUNT JLTLOOP STATOTAL RSUB TABLERESW2000 COUNTRESW1 ZEROWORD0 TOTALRESW1 ENDFIRST

29. Chap 2 Example program 1000SUMSTART4000 1000FIRSTLDXZERO 1003LDAZERO 1006LOOPADDTABLE,X 1009TIXCOUNT 100CJLTLOOP 100FSTATOTAL 1012RSUB 1015TABLERESW2000 1018COUNTRESW1 101BZEROWORD0 101ETOTALRESW1 1021ENDFIRST

30. Chap 2 OPTAB MnemonicOpcode LDX04 LDA00 ADD18 STA0C RSUB4C TIX2C JLT38 SYMTAB NameLOCCTR FIRST1000 LOOP1006 TABLE1015 COUNT1018 ZERO101B TOTAL101E

31. Chap 2 n Machine Dependent Assembler Features u instruction formats and addressing modes u program relocation n Machine Independent Assembler Features u literals u symbol-defining statements u expressions u program blocks u control sections and program linking

32. Machine-dependent Assembler Features Sec. 2-2 n Instruction formats and addressing modes n Program relocation

33. Chap 2 Instruction Format and Addressing Mode n SIC/XE u PC-relative or Base-relative addressing: op m u Indirect addressing:(avoids another instruction)op @m u Immediate addressing(no memory fetch) op #c u Extended format: +op m (programs responsibility to specify this mode when required) u Index addressing: op m,x u register-to-register instructions in place of reg-to-memory u larger memory -> multi-programming (program allocation) u BASE is used in conjunction with base relative address

34. An SIC/XE Example

37. Chap 2 PC-Relative Addressing Modes n PC-relative u 100000FIRSTSTLRETADR17202D (14) 16 1 1 0 0 1 0(02D) 16 F displacement= RETADR - PC = 30-3 = 2D u 400017JCLOOP3F2FEC (3C) 16 1 1 0 0 1 0(FEC) 16 F displacement= CLOOP-PC= 6 - 1A= -14= FEC

38. Chap 2 Base-Relative Addressing Modes n Base-relative u base register is under the control of the programmer u 12LDB#LENGTH u 13BASELENGTH u 160104ESTCHBUFFER, X57C003 ( 54 ) 16 1 1 1 1 0 0( 003 ) 16 (54) 1 1 1 0 1 0 0036-1051= -101B 16 F displacement= BUFFER - B = 0036 - 0033 = 3 u NOBASE is used to inform the assembler that the contents of the base register no longer be relied upon for addressing

39. Chap 2 Immediate Address Translation n Immediate addressing u 550020LDA#3010003 ( 00 ) 16 0 1 0 0 0 0( 003 ) 16 u 133103C +LDT#409675101000 ( 74 ) 16 0 1 0 0 0 1( 01000 ) 16

40. Chap 2 Immediate Address Translation (Cont.) n Immediate addressing u 120003LDB#LENGTH69202D ( 68) 16 0 1 0 0 1 0( 02D ) 16 ( 68) 16 0 1 0 0 0 0 ( 033) 16 690033 F the immediate operand is the symbol LENGTH F the address of this symbol LENGTH is loaded into register B F LENGTH=0033=PC+displacement=0006+02D F if immediate mode is specified, the target address becomes the operand

41. Chap 2 Indirect Address Translation n Indirect addressing u target addressing is computed as usual (PC- relative or BASE-relative) u only the n bit is set to 1 u 70002AJ@RETADR3E2003 ( 3C ) 16 1 0 0 0 1 0( 003 ) 16 F TA=RETADR=0030 F TA=(PC)+disp=002D+0003

42. Chap 2 Program Relocation n Example u Absolute program, starting address 1000 e.g. 55101BLDATHREE00102D u Relocate the program to 2000 e.g. 55101BLDATHREE00202D u Each Absolute address should be modified n Example u Except for absolute address, the rest of the instructions need not be modified F not a memory address (immediate addressing) F PC-relative, Base-relative u The only parts of the program that require modification at load time are those that specify direct addresses

43. Chap 2 Example

44. Relocatable Program n Modification record u Col 1M u Col 2-7 Starting location of the address field to be modified, relative to the beginning of the program u Col 8-9 length of the address field to be modified, in half- bytes

45. Chap 2 Object Code

46. Machine-Independent Assembler Features Literals Symbol Defining Statement Expressions Program Blocks Control Sections and Program Linking

47. Chap 2 Literals n Design idea u Let programmers to be able to write the value of a constant operand as a part of the instruction that uses it. u This avoids having to define the constant elsewhere in the program and make up a label for it. n Example program Example program u e.g. 45001AENDFIL LDA =C’EOF’032010 u 93 LTORG u 002D* =C’EOF’454F46 u e.g. 2151062WLOOP TD =X’05’E32011

48. Chap 2 Literals vs. Immediate Operands n Immediate Operands u The operand value is assembled as part of the machine instruction u e.g. 550020LDA#3010003 n Literals u The assembler generates the specified value as a constant at some other memory location u e.g. 45001AENDFIL LDA=C’EOF’032010 n Compare (Fig. 2.6) u e.g. 45001AENDFILLDAEOF032010 u 80002DEOFBYTEC’EOF’454F46

49. Chap 2 Literal - Implementation (1/3) Literal - Implementation (1/3) n Literal pools u Normally literals are placed into a pool at the end of the program F see Fig. 2ss fig\2.10.doc.10 (END statement)ss fig\2.10.doc u In some cases, it is desirable to place literals into a pool at some other location in the object program F assembler directive LTORG F reason: keep the literal operand close to the instruction

50. Chap 2 Literal - Implementation (2/3) n Duplicate literals u e.g. 2151062WLOOPTD=X’05’ u e.g. 230106BWD=X’05’ u The assemblers should recognize duplicate literals and store only one copy of the specified data value F Comparison of the defining expression Same literal name with different value, e.g. LOCCTR=* F Comparison of the generated data value The benefits of using generate data value are usually not great enough to justify the additional complexity in the assembler

51. Chap 2 Literal - Implementation (3/3) n LITTAB u literal name, the operand value and length, the address assigned to the operand n Pass 1 u build LITTAB with literal name, operand value and length, leaving the address unassigned u when LTORG statement is encountered, assign an address to each literal not yet assigned an address n Pass 2 u search LITTAB for each literal operand encountered u generate data values using BYTE or WORD statements u generate modification record for literals that represent an address in the program

52. Chap 2 Symbol-Defining Statements n Labels on instructions or data areas u the value of such a label is the address assigned to the statement n Defining symbols u symbolEQUvalue u value can be:  constant,  other symbol,  expression u making the source program easier to understand u no forward reference

53. Chap 2 Symbol-Defining Statements n Example 1 u MAXLENEQU4096 u +LDT#MAXLEN n Example 2 (Many general purpose registers) u BASEEQUR1 u COUNTEQUR2 u INDEXEQU R3 n Example 3 u MAXLENEQUBUFEND-BUFFER +LDT#4096

54. Chap 2 ORG (origin) n Indirectly assign values to symbols n Reset the location counter to the specified value F ORGvalue n Value can be:  constant,  other symbol,  expression n No forward reference n Example u SYMBOL: 6bytes u VALUE: 1word u FLAGS: 2bytes u LDAVALUE, X

55. Chap 2 ORG Example n Using EQU statements u STABRESB1100 u SYMBOLEQUSTAB u VALUEEQUSTAB+6 u FLAGEQUSTAB+9 n Using ORG statements u STABRESB1100 u ORGSTAB u SYMBOLRESB6 u VALUERESW1 u FLAGSRESB2 u ORGSTAB+1100

56. Chap 2 Expressions n Expressions can be classified as absolute expressions or relative expressions u MAXLENEQUBUFEND-BUFFER u BUFEND and BUFFER both are relative terms, representing addresses within the program u However the expression BUFEND-BUFFER represents an absolute value n When relative terms are paired with opposite signs, the dependency on the program starting address is canceled out; the result is an absolute value

57. Chap 2 SYMTAB n None of the relative terms may enter into a multiplication or division operation n Errors: u BUFEND+BUFFER u 100-BUFFER u 3*BUFFER n The type of an expression u keep track of the types of all symbols defined in the program

58. Chap 2 Example 2.9 SYMTABLITTAB

59. Chap 2 Program Blocks n Program blocks u refer to segments of code that are rearranged within a single object program unit u USE [blockname] u Default block u Example: Figure 2.11Figure 2.11 u Each program block may actually contain several separate segments of the source program

60. Chap 2 Program Blocks - Implementation n Pass 1 u each program block has a separate location counter u each label is assigned an address that is relative to the start of the block that contains it u at the end of Pass 1, the latest value of the location counter for each block indicates the length of that block u the assembler can then assign to each block a starting address in the object program n Pass 2 u The address of each symbol can be computed by adding the assigned block starting address and the relative address of the symbol to that block

61. Chap 2 FigureFigure 2.12 Figure n Each source line is given a relative address assigned and a block number(end of pass1) n For absolute symbol, there is no block number u line 107 n Example u 2000060LDALENGTH032060 u LENGTH=(Block 1)+0003= 0066+0003= 0069 u LOCCTR=(Block 0)+0009= 0009

62. Chap 2 Program Readability n Program readability u No extended format instructions on lines 15, 35, 65 u No needs for base relative addressing (line 13, 14) u LTORG is used to make sure the literals are placed ahead of any large data areas (line 253) n Object code u It is not necessary to physically rearrange the generated code in the object program u see Fig. 2.13, Fig. 2.14Fig. 2.13Fig. 2.14

63. Chap 2

64. Control Sections and Program Linking n Control Sections u are most often used for subroutines or other logical subdivisions of a program u the programmer can assemble, load, and manipulate each of these control sections separately u instruction in one control section may need to refer to instructions or data located in another section u because of this, there should be some means for linking control sections together u Fig. 2.15, 2.162.152.16

65. Chap 2 External Definition and References n External definition u EXTDEF name [, name] u EXTDEF names symbols that are defined in this control section and may be used by other sections n External reference u EXTREF name [,name] u EXTREF names symbols that are used in this control section and are defined elsewhere n Example u 15 0003 CLOOP+JSUB RDREC 4B100000 u 160 0017+STCH BUFFER,X 57900000 u 190 0028 MAXLEN WORD BUFEND-BUFFER000000

66. Chap 2 Implementation n The assembler must include information in the object program that will cause the loader to insert proper values where they are required n Define record u Col. 1D u Col. 2-7Name of external symbol defined in this control section u Col. 8-13Relative address within this control section (hexadeccimal) u Col.14-73 Repeat information in Col. 2-13 for other external symbols n Refer record u Col. 1D u Col. 2-7Name of external symbol referred to in this control section u Col. 8-73Name of other external reference symbols

67. Chap 2 Modification Record n Modification record u Col. 1M u Col. 2-7Starting address of the field to be modified (hexiadecimal) u Col. 8-9Length of the field to be modified, in half-bytes (hexadeccimal) u Col.11-16 External symbol whose value is to be added to or subtracted from the indicated field u Note: control section name is automatically an external symbol, i.e. it is available for use in Modification records. n Example u Figure 2.17 u M00000405+RDREC u M00000705+COPY

68. Chap 2 External References in Expression n Earlier definitions u required all of the relative terms be paired in an expression (an absolute expression), or that all except one be paired (a relative expression) n New restriction u Both terms in each pair must be relative within the same control section u Ex: BUFEND-BUFFER u Ex: RDREC-COPY n In general, the assembler cannot determine whether or not the expression is legal at assembly time. This work will be handled by a linking loader.

69. Assembler Design Options One-pass assemblers Multi-pass assemblers Two-pass assembler with overlay structure

70. Chap 2 Two-Pass Assembler with Overlay Structure n For small memory u pass 1 and pass 2 are never required at the same time u three segments F root: driver program and shared tables and subroutines F pass 1 F pass 2 u tree structure u overlay program

71. Chap 2 One-Pass Assemblers n Main problem u forward references F data items F labels on instructions n Solution u data items: require all such areas be defined before they are referenced u labels on instructions: no good solution

72. Chap 2 One-Pass Assemblers n Main Problem u forward reference F data items F labels on instructions n Two types of one-pass assembler u load-and-go F produces object code directly in memory for immediate execution u the other F produces usual kind of object code for later execution

73. Chap 2 Load-and-go Assembler n Characteristics u Useful for program development and testing u Avoids the overhead of writing the object program out and reading it back u Both one-pass and two-pass assemblers can be designed as load-and-go. u However one-pass also avoids the over head of an additional pass over the source program u For a load-and-go assembler, the actual address must be known at assembly time, we can use an absolute program

74. Chap 2 Forward Reference in One-pass Assembler n For any symbol that has not yet been defined 1. omit the address translation 2. insert the symbol into SYMTAB, and mark this symbol undefined 3. the address that refers to the undefined symbol is added to a list of forward references associated with the symbol table entry 4. when the definition for a symbol is encountered, the proper address for the symbol is then inserted into any instructions previous generated according to the forward reference list

75. Chap 2 Load-and-go Assembler (Cont.) n At the end of the program u any SYMTAB entries that are still marked with * indicate undefined symbols u search SYMTAB for the symbol named in the END statement and jump to this location to begin execution n The actual starting address must be specified at assembly time n Example u Figure 2.18, 2.192.182.19

76. Chap 2 Producing Object Code n When external working-storage devices are not available or too slow (for the intermediate file between the two passes n Solution: u When definition of a symbol is encountered, the assembler must generate another Tex record with the correct operand address u The loader is used to complete forward references that could not be handled by the assembler u The object program records must be kept in their original order when they are presented to the loader n Example: Figure 2.202.20

77. Chap 2 Multi-Pass Assemblers n Restriction on EQU and ORG u no forward reference, since symbols’ value can’t be defined during the first pass n Example u Use link list to keep track of whose value depend on an undefined symbol n Figure 2.212.21