Unit- 2 Assembler Design
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UNIT- 2 Assembler Design Assembler is system software which is used to convert an assembly language program to its equivalent object code. The input to the assembler is a source code written in assembly language (using mnemonics) and the output is the object code. The design of an assembler depends upon the machine architecture as the language used is mnemonic language. Basic Assembler Functions: The basic assembler functions are:
So for the design of the assembler we need to concentrate on the machine architecture of the SIC/XE machine. We need to identify the algorithms and the various data structures to be used. According to the above required steps for assembling the assembler also has to handle assembler directives, these do not generate the object code but directs the assembler to perform certain operation. These directives are:
End of file: a zero length record The assembler design can be done:
Single-pass Assembler: In this case the whole process of scanning, parsing, and object code conversion is done in single pass. The only problem with this method is resolving forward reference. This is shown with an example below: 10 1000 FIRST STL RETADR 141033 -- -- -- -- 95 1033 RETADR RESW 1 In the above example in line number 10 the instruction STL will store the linkage register with the contents of RETADR. But during the processing of this instruction the value of this symbol is not known as it is defined at the line number 95. Since I single-pass assembler the scanning, parsing and object code conversion happens simultaneously. The instruction is fetched; it is scanned for tokens, parsed for syntax and semantic validity. If it valid then it has to be converted to its equivalent object code. For this the object code is generated for the opcode STL and the value for the symbol RETADR need to be added, which is not available. Due to this reason usually the design is done in two passes. So a multi-pass assembler resolves the forward references and then converts into the object code. Hence the process of the multi-pass assembler can be as follows: Pass-1
Pass-2
Assembler Design: The most important things which need to be concentrated is the generation of Symbol table and resolving forward references.
Example Program: The example program considered here has a main module, two subroutines
- 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
-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
-RDREC, WRREC -Save link register first before nested jump 
The first column shows the line number for that instruction, second column shows the addresses allocated to each instruction. The third column indicates the labels given to the statement, and is followed by the instruction consisting of opcode and operand. The last column gives the equivalent object code. The object code later will be loaded into memory for execution. The simple object program we use contains three types of records:
- Col. 1 H - Col. 2~7 Program name - Col. 8~13 Starting address of object program (hex) - Col. 14~19 Length of object program in bytes (hex)
- Col. 1 T - Col. 2~7 Starting address for object code in this record (hex) - Col. 8~9 Length of object code in this record in bytes (hex) - Col. 10~69 Object code, represented in hex (2 col. per byte)
- Col.1 E
Object code for the example program: Some of the features in the program depend on the architecture of the machine. If the program is for SIC machine, then we have only limited instruction formats and hence limited addressing modes. We have only single operand instructions. The operand is always a memory reference. Anything to be fetched from memory requires more time. Hence the improved version of SIC/XE machine provides more instruction formats and hence more addressing modes. The moment we change the machine architecture the availability of number of instruction formats and the addressing modes changes. Therefore the design usually requires considering two things: Machine-dependent features and Machine-independent features. Machine-Dependent Features:
Instruction formats and Addressing Modes The instruction formats depend on the memory organization and the size of the memory. In SIC machine the memory is byte addressable. Word size is 3 bytes. So the size of the memory is 212 bytes. Accordingly it supports only one instruction format. It has only two registers: register A and Index register. Therefore the addressing modes supported by this architecture are direct, indirect, and indexed. Whereas the memory of a SIC/XE machine is 220 bytes (1 MB). This supports four different types of instruction types, they are:
During pass 1 the registers can be entered as part of the symbol table itself. The value for these registers is their equivalent numeric codes. During pass 2, these values are assembled along with the mnemonics object code. If required a separate table can be created with the register names and their equivalent numeric values.
In SIC/XE machine there are four instruction formats and five addressing modes. Among the instruction formats, format -3 and format-4 instructions are Register-Memory type of instruction. One of the operand is always in a register and the other operand is in the memory. The addressing mode tells us the way in which the operand from the memory is to be fetched. There are two ways: Program-counter relative and Base-relative. This addressing mode can be represented by either using format-3 type or format-4 type of instruction format. In format-3, the instruction has the opcode followed by a 12-bit displacement value in the address field. Where as in format-4 the instruction contains the mnemonic code followed by a 20-bit displacement value in the address field.
TA = (base) + displacement value This addressing mode is used when the range of displacement value is not sufficient. Hence the operand is not relative to the instruction as in PC-relative addressing mode. Whenever this mode is used it is indicated by using a directive BASE. The moment the assembler encounters this directive the next instruction uses base-relative addressing mode to calculate the target address of the operand. When NOBASE directive is used then it indicates the base register is no more used to calculate the target address of the operand. Assembler first chooses PC-relative, when the displacement field is not enough it uses Base-relative. LDB #LENGTH (instruction) BASE LENGTH (directive) : NOBASE
For example: 12 0003 LDB #LENGTH 69202D 13 BASE LENGTH : :
100 0033 LENGTH RESW 1 105 0036 BUFFER RESB 4096 : : 160 104E STCH BUFFER, X 57C003 165 1051 TIXR T B850 In the above example the use of directive BASE indicates that Base-relative addressing mode is to be used to calculate the target address. PC-relative is no longer used. The value of the LENGTH is stored in the base register. If PC-relative is used then the target address calculated is: The LDB instruction loads the value of length in the base register which 0033. BASE directive explicitly tells the assembler that it has the value of LENGTH. BUFFER is at location (0036)16 (B) = (0033)16 disp = 0036 – 0033 = (0003)16 
20 000A LDA LENGTH 032026 : : 175 1056 EXIT STX LENGTH 134000 Consider Line 175. If we use PC-relative Disp = TA – (PC) = 0033 –1059 = EFDA PC relative is no longer applicable, so we try to use BASE relative addressing mode.
In this mode no memory reference is involved. If immediate mode is used the target address is the operand itself. 
If the symbol is referred in the instruction as the immediate operand then it is immediate with PC-relative mode as shown in the example below: 
5. Indirect and PC-relative mode: In this type of instruction the symbol used in the instruction is the address of the location which contains the address of the operand. The address of this is found using PC-relative addressing mode. For example: 
The instruction jumps the control to the address location RETADR which in turn has the address of the operand. If address of RETADR is 0030, the target address is then 0003 as calculated above. Program Relocation Sometimes it is required to load and run several programs at the same time. The system must be able to load these programs wherever there is place in the memory. Therefore the exact starting is not known until the load time. Absolute Program In this the address is mentioned during assembling itself. This is called Absolute Assembly. Consider the instruction: 55 101B LDA THREE 00102D This statement says that the register A is loaded with the value stored at location 102D. Suppose it is decided to load and execute the program at location 2000 instead of location 1000. Then at address 102D the required value which needs to be loaded in the register A is no more available. The address also gets changed relative to the displacement of the program. Hence we need to make some changes in the address portion of the instruction so that we can load and execute the program at location 2000. Apart from the instruction which will undergo a change in their operand address value as the program load address changes. There exist some parts in the program which will remain same regardless of where the program is being loaded. Since assembler will not know actual location where the program will get loaded, it cannot make the necessary changes in the addresses used in the program. However, the assembler identifies for the loader those parts of the program which need modification. An object program that has the information necessary to perform this kind of modification is called the relocatable program.
A control section is a part of the program that maintains its identity after assembly; each control section can be loaded and relocated independently of the others. Different control sections are most often used for subroutines or other logical subdivisions. The programmer can assemble, load, and manipulate each of these control sections separately. Because of this, there should be some means for linking control sections together. For example, instructions in one control section may refer to the data or instructions of other control sections. Since control sections are independently loaded and relocated, the assembler is unable to process these references in the usual way. Such references between different control sections are called external references. The assembler generates the information about each of the external references that will allow the loader to perform the required linking. When a program is written using multiple control sections, the beginning of each of the control section is indicated by an assembler directive
The syntax secname CSECT
Control sections differ from program blocks in that they are handled separately by the assembler. Symbols that are defined in one control section may not be used directly another control section; they must be identified as external reference for the loader to handle. The external references are indicated by two assembler directives: EXTDEF (external Definition): It is the statement in a control section, names symbols that are defined in this section but may be used by other control sections. Control section names do not need to be named in the EXTREF as they are automatically considered as external symbols. EXTREF (external Reference): It names symbols that are used in this section but are defined in some other control section. The order in which these symbols are listed is not significant. The assembler must include proper information about the external references in the object program that will cause the loader to insert the proper value where they are required. Handling External Reference Case 1 15 0003 CLOOP +JSUB RDREC 4B100000
Case 2 190 0028 MAXLEN WORD BUFEND-BUFFER 000000
Case 3 On line 107, BUFEND and BUFFER are defined in the same control section and the expression can be calculated immediately. 107 1000 MAXLEN EQU BUFEND-BUFFER
The assembler must also include information in the object program that will cause the loader to insert the proper value where they are required. The assembler maintains two new record in the object code and a changed version of modification record. Define record (EXTDEF)
Refer record (EXTREF)
Modification record
the indicated field A define record gives information about the external symbols that are defined in this control section, i.e., symbols named by EXTDEF. A refer record lists the symbols that are used as external references by the control section, i.e., symbols named by EXTREF. The new items in the modification record specify the modification to be performed: adding or subtracting the value of some external symbol. The symbol used for modification my be defined either in this control section or in another section. The object program is shown below. There is a separate object program for each of the control sections. In the Define Record and refer record the symbols named in EXTDEF and EXTREF are included. In the case of Define, the record also indicates the relative address of each external symbol within the control section. For EXTREF symbols, no address information is available. These symbols are simply named in the Refer record.
The existence of multiple control sections that can be relocated independently of one another makes the handling of expressions complicated. It is required that in an expression that all the relative terms be paired (for absolute expression), or that all except one be paired (for relative expressions). When it comes in a program having multiple control sections then we have an extended restriction that:
ASSEMBLER DESIGN Here we are discussing
One-Pass Assembler The main problem in designing the assembler using single pass was to resolve forward references. We can avoid to some extent the forward references by:
There are two types of one-pass assemblers:
Load-and-Go Assembler
Forward Reference in One-Pass Assemblers: In load-and-Go assemblers when a forward reference is encountered :
After Scanning line 40 of the program: 40 2021 J` CLOOP 302012 The status is that upto this point the symbol RREC is referred once at location 2013, ENDFIL at 201F and WRREC at location 201C. None of these symbols are defined. The figure shows that how the pending definitions along with their addresses are included in the symbol table. 
The status after scanning line 160, which has encountered the definition of RDREC and ENDFIL is as given below: If One-Pass needs to generate object code:
Multi_Pass Assembler:
ALPHA EQU BETA BETA EQU DELTA DELTA RESW 1
Implementation Issues for Modified Two-Pass Assembler: Implementation Isuues when forward referencing is encountered in Symbol Defining statements :
Important Question
Programming in machine code, by supplying the computer with the numbers of the operations it must perform, can be quite a burden, because for every operation the corresponding number must be looked up or remembered. Looking up all numbers takes a lot of time, and mis-remembering a number may introduce computer bugs. So Assembly Languages are evolved which contains mnemonic instructions corresponding to the Machine codes using which the program can be written easily. Therefore a set of mnemonics was devised. Each number was represented by an alphabetic code. So instead of entering the number corresponding to addition to add two numbers one can enter "add". Although mnemonics differ between different CPU designs some are common, for instance: "sub" (subtract), "div" (divide), "add" (add) and "mul" (multiply).
An assembler is a program that accepts an assembly language program as input and produces its machine language equivalent along with information for the loader (An Assembler translates a program written in an assembly language to it machine language equivalent)
(What is the format in which the assembly language program is written?).
Functions of a Basic Assembler
language equivalents E.g. STL -> 14 (line 10)
machine addresses E.g. RETADR -> 1033 (line 10)
representations E.g. EOF -> 454F46 (line 80)
Assembler directives are Pseudo-instructions that are not translated into machine instructions and they provide instructions to the assembler itself.
Functions of Two Pass Assembler
Assemble instructions
Contains 3 types of records: Header record: Col. 1 H
Col. 2-7 Program name Col. 8-13 Starting address (hex) Col. 14-19 Length of object program in bytes (hex)
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 (hex) (2 columns per byte)
Col.1 E
Col.2~7 Address of first executable instruction (hex) (END program_name)
Forward reference is a reference to a label that is defined later in the program. Example 10 STL RETADR o RETADR is not yet defined when we encounter STL instruction o So it is called forward reference..
Line Loc Source statement Object code 5 1000 COPY START 1000 10 1000 FIRST STL RETADR 141033 15 1003 CLOOP JSUB RDREC 482039 20 1006 LDA LENGTH 001036 25 1009 COMP ZERO 281030 30 100C JEQ ENDFIL 301015 35 100F JSUB WRREC 482061 40 1012 J CLOOP 3C1003 45 1015 ENDFIL LDA EOF 00102A 50 1018 STA BUFFER 0C1039 55 101B LDA THREE 00102D 60 101E STA LENGTH 0C1036 65 1021 JSUB WRREC 482061 70 1024 LDL RETADR 081033 75 1027 RSUB 4C0000 80 102A EOF BYTE C’EOF’ 454F46 85 102D THREE WORD 3 000003 90 1030 ZERO WORD 0 000000 95 1033 RETADR RESW 1 100 1036 LENGTH RESW 1 105 1039 BUFFER RESB 4096 110 . 115 . SUBROUTINE TO READ RECORD INTO BUFFER 120 . 125 2039 RDREC LDX ZERO 041030 130 203C LDA ZERO 001030 135 203F RLOOP TD INPUT E0205D 140 2042 JEQ RLOOP 30203D 145 2045 RD INPUT D8205D 150 2048 COMP ZERO 281030 155 204B JEQ EXIT 302057 160 204E STCH BUFFER,X 549039 165 2051 TIX MAXLEN 2C205E 170 2054 JLT RLOOP 38203F 175 2057 EXIT STX LENGTH 101036 180 205A RSUB 4C0000 185 205D INPUT BYTE X’F1’ F1 190 205E MAXLEN WORD 4096 001000 195 . 200 . SUBROUTINE TO WRITE RECORD FROM BUFFER 205 . 210 2061 WRREC LDX ZERO 041030 215 2064 WLOOP TD OUTPUT E02079 220 2067 JEQ WLOOP 302064 225 206A LDCH BUFFER,X 509039 230 206D WD OUTPUT DC2079 235 2070 TIX LENGTH 2C1036 240 2073 JLT WLOOP 382064 245 2076 RSUB 4C0000 250 2079 OUTPUT BYTE X’05’ 05 255 END FIRST . 10) Write an Algorithm for pass 1 of SIC Assembler. 
11) Write an algorithm for pass 2 of SIC assembler. 
Data Structures: Operation Code Table (OPTAB) Symbol Table (SYMTAB) Location Counter(LOCCTR)
Label name and its value (address) May also include flag (type, length) etc.
Pass 1: labels are entered into SYMTAB with their address (from LOCCTR) as they are encountered in the source program Pass 2: symbols used as operands are looked up in SYMTAB to obtain the address to be inserted in the assembled instruction
Dynamic table (insert, delete, search)
Hash table for efficiency of insertion and retrieval
SYMBOL TABLE(SYMTAB)
Location Counter
Machine-dependent features of assemblers Features of the SIC/XE machine Programming features.
Assembling features.
Program relocation
Program Relocation
Program Blocks
Program Blocks - Implementation
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