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:
Translating mnemonic language code to its equivalent object code.
Assigning machine addresses to symbolic labels.
The design of assembler can be to perform the following:
Convert mnemonic operation codes to their machine language equivalents
Convert symbolic operands to their equivalent machine addresses
Decide the proper instruction format Convert the data constants to internal machine representations
Write the object program and the assembly listing
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:
SIC Assembler Directive:
START: Specify name & starting address.
END: End of the program, specify the first execution instruction.
BYTE, WORD, RESB, RESW
End of record: a null char(00)
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:
Assign addresses to all the statements
Save the addresses assigned to all labels to be used in Pass-2
Perform some processing of assembler directives such as RESW, RESB to find the length of data areas for assigning the address values.
Defines the symbols in the symbol table(generate the symbol table)
Assemble the instructions (translating operation codes and looking up addresses).
The most important things which need to be concentrated is the generation of Symbol table and resolving forward references.
This is created during pass 1
All the labels of the instructions are symbols
Table has entry for symbol name, address value.
Symbols that are defined in the later part of the program are called forward referencing.
There will not be any address value for such symbols in the symbol table in pass
The example program considered here has a main module, two subroutines
Purpose of example program
- 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
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)
-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
Col.2~7 Address of first executable instruction in object program (hex) “^” is only for separation only
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.
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:
1 byte instruction
2 byte instruction
3 byte instruction
4 byte instruction
Instructions can be:
Instructions involving register to register
Instructions with one operand in memory, the other in Accumulator (Single operand instruction)
Extended instruction format
Addressing Modes are:
Index Addressing(SIC): Opcode m, x
Indirect Addressing: Opcode @m
PC-relative: Opcode m
Base relative: Opcode m
Immediate addressing: Opcode #c
Translations for the Instruction involving Register-Register addressing mode:
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.
2. Program-Counter Relative: In this usually format-3 instruction format is used. The instruction contains the opcode followed by a 12-bit displacement value. The range of displacement values are from 0 -2048. This displacement (should be small enough to fit in a 12-bit field) value is added to the current contents of the program counter to get the target address of the operand required by the instruction. This is relative way of calculating the address of the operand relative to the program counter. Hence the displacement of the operand is relative to the current program counter value. The following example shows how the address is calculated:
3. Base-Relative Addressing Mode: in this mode the base register is used to mention the displacement value. Therefore the target address is
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.
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.
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.
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
assembler directive: CSECT
separate location counter for each control section
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
15 0003 CLOOP +JSUB RDREC 4B100000
The operand RDREC is an external reference.
The assembler has no idea where RDREC is
inserts an address of zero
can only use extended format to provide enough room (that is, relative addressing for external reference is invalid)
The assembler generates information for each external reference that will allow the loader to perform the required linking.
190 0028 MAXLEN WORD BUFEND-BUFFER 000000
There are two external references in the expression, BUFEND and BUFFER.
Subtract from this data area the address of BUFFER
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
Object Code for the example program:
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)
Col. 1 D
Col. 2-7 Name of external symbol defined in this control section
Col. 8-13 Relative address within this control section (hexadecimal)
Col.14-73 Repeat information in Col. 2-13 for other external symbols
Refer record (EXTREF)
Col. 1 R
Col. 2-7 Name of external symbol referred to in this control section
Col. 8-73 Name of other external reference symbols
Col. 1 M
Col. 2-7 Starting address of the field to be modified (hexadecimal)
Col. 8-9 Length of the field to be modified, in half-bytes (hexadecimal)
Col.11-16 External symbol whose value is to be added to or subtracted from
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.
Handling Expressions in Multiple Control Sections:
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:
Both terms in each pair of an expression must be within the same control section
If two terms represent relative locations within the same control section , their difference is an absolute value (regardless of where the control section is located.
Legal: BUFEND-BUFFER (both are in the same control section)
If the terms are located in different control sections, their difference has a value that is unpredictable.
Illegal: RDREC-COPY (both are of different control section) it is the difference in the load addresses of the two control sections. This value depends on the way run-time storage is allocated; it is unlikely to be of any use.
How to enforce this restriction
When an expression involves external references, the assembler cannot determine whether or not the expression is legal.
The assembler evaluates all of the terms it can, combines these to form an initial expression value, and generates Modification records.
The loader checks the expression for errors and finishes the evaluation.
Here we are discussing
The structure and logic of one-pass assembler. These assemblers are used when it is necessary or desirable to avoid a second pass over the source program.
Notion of a multi-pass assembler, an extension of two-pass assembler that allows an assembler to handle forward references during symbol definition.
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:
Eliminating forward reference to data items, by defining all the storage reservation statements at the beginning of the program rather at the end.
Unfortunately, forward reference to labels on the instructions cannot be avoided. (forward jumping)
To provide some provision for handling forward references by prohibiting forward references to data items.
There are two types of one-pass assemblers:
One that produces object code directly in memory for immediate execution (Load-and-go assemblers).
The other type produces the usual kind of object code for later execution.
Load-and-go assembler generates their object code in memory for immediate execution.
No object program is written out, no loader is needed.
It is useful in a system with frequent program development and testing
The efficiency of the assembly process is an important consideration.
Programs are re-assembled nearly every time they are run; efficiency of the assembly process is an important consideration.
Forward Reference in One-Pass Assemblers: In load-and-Go assemblers when a forward reference is encountered :
Omits the operand address if the symbol has not yet been defined
Enters this undefined symbol into SYMTAB and indicates that it is undefined
Adds the address of this operand address to a list of forward references associated with the SYMTAB entry
When the definition for the symbol is encountered, scans the reference list and inserts the address.
At the end of the program, reports the error if there are still SYMTAB entries indicated undefined symbols.
For Load-and-Go assembler
Search SYMTAB for the symbol named in the END statement and jumps to this location to begin execution if there is no error
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:
If the operand contains an undefined symbol, use 0 as the address and write the Text record to the object program.
Forward references are entered into lists as in the load-and-go assembler.
When the definition of a symbol is encountered, the assembler generates another Text record with the correct operand address of each entry in the reference list.
When loaded, the incorrect address 0 will be updated by the latter Text record containing the symbol definition.
For a two pass assembler, forward references in symbol definition are not allowed:
ALPHA EQU BETA
BETA EQU DELTA
DELTA RESW 1
Symbol definition must be completed in pass 1.
Prohibiting forward references in symbol definition is not a serious inconvenience.
Forward references tend to create difficulty for a person reading the program.
Implementation Issues for Modified Two-Pass Assembler:
For a forward reference in symbol definition, we store in the SYMTAB:
The symbol name
The defining expression
The number of undefined symbols in the defining expression
The undefined symbol (marked with a flag *) associated with a list of symbols depend on this undefined symbol.
When a symbol is defined, we can recursively evaluate the symbol expressions depending on the newly defined symbol.
Why an Assembly Language is needed?
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).
What is an Assembler?
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)
Explain the terms a)Label,b)Opcode,c)Operand,and d)Comment
(What is the format in which the assembly language program is written?).
The label is a symbolic name that represents the memory address of an executable statement or a variable.
The opcode (e.g. operation code) specifies the symbolic name for a machine instruction.
The directive specifies commands to the assembler about the way to assemble the program.
The operand specifies the data that is needed by a statement.
The comment provides clear explanation for a statement.