Assembler

Assemblers System Software by Leland L. Beck Chapter 2

Role of Assembler Source Program Assembler Object Code Loader Executable Code Linker

Chapter 2 -- Outline Basic Assembler Functions Machine-dependent Assembler Features Machine-independent Assembler Features Assembler Design Options

Introduction to Assemblers Fundamental functions translating mnemonic operation codes to their machine language equivalents assigning machine addresses to symbolic labels Machine dependency different machine instruction formats and codes

Example Program (Fig. 2.1) Purpose reads records from input device (code F1) copies them to output device (code 05) at the end of the file, writes EOF on the output device, then RSUB to the operating system program

Example Program (Fig. 2.1) Data transfer (RD, WD) a buffer is used to store record buffering is necessary for different I/O rates the end of each record is marked with a null character (00 16 ) the end of the file is indicated by a zero-length record Subroutines (JSUB, RSUB) RDREC, WRREC save link register first before nested jump

Assembler Directives Pseudo-Instructions Not translated into machine instructions Providing information to the assembler Basic assembler directives START END BYTE WORD RESB RESW

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

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

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 }

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’

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 }

Assembler’s functions Convert mnemonic operation codes to their machine language equivalents Convert symbolic operands to their equivalent machine addresses  Build the machine instructions in the proper format Convert the data constants to internal machine representations Write the object program and the assembly listing Break...

Example of Instruction Assemble Forward reference STCH BUFFER,X (54) 16 1 (001) 2 (039) 16 549039

Difficulties: Forward Reference Forward reference: reference to a label that is defined later in the program. Loc Label Operator Operand 1000 FIRST STL RETADR 1003 CLOOP JSUB RDREC … … … … … 1012 J CLOOP … … … … … 1033 RETADR RESW 1

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

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

Data Structures Operation Code Table (OPTAB) Symbol Table (SYMTAB) Location Counter(LOCCTR)

OPTAB (operation code table) Content menmonic, machine code (instruction format, length) etc. Characteristic static table Implementation array or hash table, easy for search

SYMTAB (symbol table) Content label name, value, flag, (type, length) etc. Characteristic dynamic table (insert, delete, search) Implementation hash table, non-random keys, hashing function COPY 1000 FIRST 1000 CLOOP 1003 ENDFIL 1015 EOF 1024 THREE 102D ZERO 1030 RETADR 1033 LENGTH 1036 BUFFER 1039 RDREC 2039

Homework #3 SUM START 4000 FIRST LDX ZERO LDA ZERO LOOP ADD TABLE,X TIX COUNT JLT LOOP STA TOTAL RSUB TABLE RESW 2000 COUNT RESW 1 ZERO WORD 0 TOTAL RESW 1 END FIRST End of Sec 2-1

Assembler Design Machine Dependent Assembler Features instruction formats and addressing modes program relocation Machine Independent Assembler Features literals symbol-defining statements expressions program blocks control sections and program linking

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

Instruction Format and Addressing Mode SIC/XE PC-relative or Base-relative addressing: op m Indirect addressing: op @m Immediate addressing: op #c Extended format: +op m Index addressing: op m,x register-to-register instructions larger memory -> multi-programming (program allocation) Example program

Translation Register translation register name (A, X, L, B, S, T, F, PC, SW) and their values (0,1, 2, 3, 4, 5, 6, 8, 9) preloaded in SYMTAB Address translation Most register-memory instructions use program counter relative or base relative addressing Format 3: 12-bit address field base-relative: 0~4095 pc-relative: -2048~2047 Format 4: 20-bit address field

PC-Relative Addressing Modes PC-relative 10 0000 FIRST STL RETADR 17202D (14) 16 1 1 0 0 1 0 (02D) 16 displacement= RETADR - PC = 30-3 = 2D 40 0017 J CLOOP 3F2FEC (3C) 16 1 1 0 0 1 0 (FEC) 16 displacement= CLOOP-PC= 6 - 1A= -14= FEC

Base-Relative Addressing Modes Base-relative base register is under the control of the programmer 12 LDB #LENGTH 13 BASE LENGTH 160 104E STCH BUFFER, X 57C003 ( 54 ) 16 1 1 1 1 0 0 ( 003 ) 16 (54) 1 1 1 0 1 0 0036-1051= -101B 16 displacement= BUFFER - B = 0036 - 0033 = 3 NOBASE is used to inform the assembler that the contents of the base register no longer be relied upon for addressing

Immediate Address Translation Immediate addressing 55 0020 LDA #3 010003 ( 00 ) 16 0 1 0 0 0 0 ( 003 ) 16 133 103C +LDT #4096 75101000 ( 74 ) 16 0 1 0 0 0 1 ( 01000 ) 16

Immediate Address Translation (Cont.) Immediate addressing 12 0003 LDB #LENGTH 69202D ( 68) 16 0 1 0 0 1 0 ( 02D ) 16 ( 68) 16 0 1 0 0 0 0 ( 033) 16 690033 the immediate operand is the symbol LENGTH the address of this symbol LENGTH is loaded into register B LENGTH=0033=PC+displacement=0006+02D if immediate mode is specified, the target address becomes the operand

Indirect Address Translation Indirect addressing target addressing is computed as usual (PC-relative or BASE-relative) only the n bit is set to 1 70 002A J @RETADR 3E2003 ( 3C ) 16 1 0 0 0 1 0 ( 003 ) 16 TA=RETADR=0030 TA=(PC)+disp=002D+0003

Program Relocation Example Fig. 2.1 Absolute program , starting address 1000 e.g. 55 101B LDA THREE 00102D Relocate the program to 2000 e.g. 55 101B LDA THREE 00202D Each Absolute address should be modified Example Fig. 2.5 : Except for absolute address, the rest of the instructions need not be modified not a memory address (immediate addressing) PC-relative, Base-relative The only parts of the program that require modification at load time are those that specify direct addresses

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

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

Literals Design idea Let programmers to be able to write the value of a constant operand as a part of the instruction that uses it. This avoids having to define the constant elsewhere in the program and make up a label for it. Example e.g. 45 001A ENDFIL LDA =C’EOF’ 032010 93 LTORG 002D * =C’EOF’ 454F46 e.g. 215 1062 WLOOP TD =X’05’ E32011

Literals vs. Immediate Operands Immediate Operands The operand value is assembled as part of the machine instruction e.g. 55 0020 LDA #3 010003 Literals The assembler generates the specified value as a constant at some other memory location e.g. 45 001A ENDFIL LDA =C’EOF’ 032010 Compare (Fig. 2.6) e.g. 45 001A ENDFIL LDA EOF 032010 80 002D EOF BYTE C’EOF’ 454F46

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

Literal - Implementation (2/3) Duplicate literals e.g. 215 1062 WLOOP TD =X’05’ e.g. 230 106B WD =X’05’ The assemblers should recognize duplicate literals and store only one copy of the specified data value Comparison of the defining expression Same literal name with different value, e.g. LOCCTR=* 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

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

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

Symbol-Defining Statements Example 1 MAXLEN EQU 4096 +LDT #MAXLEN Example 2 (Many general purpose registers) BASE EQU R1 COUNT EQU R2 INDEX EQU R3 Example 3 MAXLEN EQU BUFEND-BUFFER +LDT #4096

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

ORG Example Using EQU statements STAB RESB 1100 SYMBOL EQU STAB VALUE EQU STAB+6 FLAG EQU STAB+9 Using ORG statements STAB RESB 1100 ORG STAB SYMBOL RESB 6 VALUE RESW 1 FLAGS RESB 2 ORG STAB+1100

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

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

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

Program Blocks - Implementation Pass 1 each program block has a separate location counter each label is assigned an address that is relative to the start of the block that contains it at the end of Pass 1, the latest value of the location counter for each block indicates the length of that block the assembler can then assign to each block a starting address in the object program Pass 2 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

Figure 2.12 Each source line is given a relative address assigned and a block number For absolute symbol, there is no block number line 107 Example 20 0006 0 LDA LENGTH 032060 LENGTH=(Block 1)+0003= 0066+0003= 0069 LOCCTR=(Block 0)+0009= 0009

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

Control Sections and Program Linking Control Sections are most often used for subroutines or other logical subdivisions of a program the programmer can assemble, load, and manipulate each of these control sections separately instruction in one control section may need to refer to instructions or data located in another section because of this, there should be some means for linking control sections together Fig. 2.15 , 2.16

External Definition and References External definition EXTDEF name [, name] EXTDEF names symbols that are defined in this control section and may be used by other sections External reference EXTREF name [,name] EXTREF names symbols that are used in this control section and are defined elsewhere Example 15 0003 CLOOP +JSUB RDREC 4B100000 160 0017 +STCH BUFFER,X 57900000 190 0028 MAXLEN WORD BUFEND-BUFFER 000000

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

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

External References in Expression Earlier definitions required all of the relative terms be paired in an expression (an absolute expression), or that all except one be paired (a relative expression) New restriction Both terms in each pair must be relative within the same control section Ex: BUFEND-BUFFER Ex: RDREC-COPY 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.

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

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

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

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

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

Forward Reference in One-pass Assembler 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

Load-and-go Assembler (Cont.) At the end of the program any SYMTAB entries that are still marked with * indicate undefined symbols search SYMTAB for the symbol named in the END statement and jump to this location to begin execution The actual starting address must be specified at assembly time Example Figure 2.18 , 2.19

Producing Object Code When external working-storage devices are not available or too slow (for the intermediate file between the two passes Solution: When definition of a symbol is encountered, the assembler must generate another Tex record with the correct operand address The loader is used to complete forward references that could not be handled by the assembler The object program records must be kept in their original order when they are presented to the loader Example: Figure 2.20

Multi-Pass Assemblers Restriction on EQU and ORG no forward reference, since symbols’ value can’t be defined during the first pass Example Use link list to keep track of whose value depend on an undefined symbol Figure 2.21

Assembler

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