4.1 LINKING AND RELOCATION
Figure 4.1 Creation and execution of a program
The DOS linking program links the different object modules of a source program and function library routines to generate an integrated executable code of the source program. The main input to the linker is the .OBJ file that contains the object modules of the source programs. Other supporting information may be obtained from the files generated by the MASM.
The linker program is invoked using the following options.
C> LINK
or
C>LINK MS.OBJ
The .OBJ extension is a must for a file to be accepted by the LINK as a valid object file. The first object may generate a display asking for the object file, list file and libraries as inputs and an expected name of the .EXE file to be generated. The output of the link program is an executable file with the entered filename and .EXE extension. This executable filename can further be entered at the DOS prompt to execute the file.
In the advanced version of the MASM, the complete procedure of assembling and linking is combined under a single menu invokable compile function. The recent versions of MASM have much more sophisticated and user-friendly facilities and options. A linker links the machine codes with the other required assembled codes. Linking is necessary because of the number of codes to be linked for the final binary file.
The linked file in binary for run on a computer is commonly known as executable file or simply ‘.exe.’ file. After linking, there has to be re-allocation of the sequences of placing the codes before actually placement of the codes in the memory. The loader program performs the task of reallocating the codes after finding the physical RAM addresses available at a given instant.
The DOS linking program links the different object modules of a source program and function library routines to generate an integrated executable code of the source program. The main input to the linker is the .OBJ file that contains the object modules of the source programs. Other supporting information may be obtained from the files generated by the MASM.
The linked file in binary for run on a computer is commonly known as executable file or simply ‘.exe.’ file. After linking, there has to be re-allocation of the sequences of placing the codes before actually placement of the codes in the memory. The loader program performs the task of reallocating the codes after finding the physical RAM addresses available at a given instant.
The loader is a part of the operating system and places codes into the memory after reading the ‘.exe’ file. This step is necessary because the available memory addresses may not start from 0x0000, and binary codes have to be loaded at the different addresses during the run. The loader finds the appropriate start address.
In a computer, the loader is used and it loads into a section of RAM the program that is ready to run. A program called locator reallocates the linked file and creates a file for permanent location of codes in a standard format.
4.1.1 Segment combination
In addition to the linker commands, the assembler provides a means of regulating the way segments in different object modules are organized by the linker. Segments with same name are joined together by using the modifiers attached to the SEGMENT directives. SEGMENT directive may have the form
Segment name SEGMENT Combination-type
where the combine-type indicates how the segment is to be located within the load module. Segments that have different names cannot be combined and segments with the same name but no combine-type will cause a linker error. The possible combine-types are:
PUBLIC – If the segments in different modules have the same name and combine-type PUBLIC, then they are concatenated into a single element in the load module. The ordering in the concatenation is specified by the linker command.
COMMON – If the segments in different object modules have the same name and the combine-type is COMMON, then they are overlaid so that they have the same starting address. The length of the common segment is that of the longest segment being overlaid.
STACK – If segments in different object modules have the same name and the combine- type STACK, then they become one segment whose length is the sum of the lengths of the individually specified segments. In effect, they are combined to form one large stack
AT – The AT combine-type is followed by an expression that evaluates to a constant which is to be the segment address. It allows the user to specify the exact location of the segment in memory.
MEMORY – This combine-type causes the segment to be placed at the last of the load module. If more than one segment with the MEMORY combine-type is being linked, only the first one will be treated as having the MEMORY combine type; the others will be overlaid as if they had COMMON combine-type.
4.1.2 Access to External Identifiers
If an identifier is defined in an object module, then it is said to be a local (or internal ) identifier relative to the module. If it is not defined in the module but is defined in one of the other modules being linked, then it is referred to as an external (or global) identifier relative to the module.In order to permit other object modules to reference some of the identifiers in a given module, the given module must include a list of the identifiers to which it will allow access. Therefore, each module in multi-module programs may contain two lists, one containing the external identifiers that can be referred to by other modules.
Two lists are implemented by the EXTRN and PUBLIC directives, which have the forms:
EXTRN Identifier: Type…, Identifier: Type
and
PUBLIC Identifier,…, Identifier
where the identifiers are the variables and labels being declared or as being available to other modules.
The assembler must know the type of all external identifiers before it can generate the proper machine code, a type specifier must be associated with each identifier in an EXTRN statement.
For a variable the type may be BYTE, WORD, or DWORD and for a label it may be NEAR or FAR.
One of the primary tasks of the linker is to verify that every identifier appearing in an EXTRN statement is matched by one in a PUBLIC statement. If this is not the case, then there will be an undefined reference and a linker error will occur.
The offsets for the local identifier will be inserted by the assembler, but the offsets for the external identifiers and all segment addresses must be inserted by the linking process.The offsets associated with all external references can be assigned once all of the object modules have been found and their external symbol tables have been examined.
The assignment of the segment addresses is called relocation and is done after the linking process has determined exactly where each segment is to be put in memory.
4.2 STACKS
The stack is a block of memory that may be used for temporarily storing the contents of the registers inside the CPU. It is a top-down data structure whose elements are accessed using the stack pointer (SP) which gets decremented by two as we store a data word into the stack and gets incremented by two as we retrieve a data word from the stack back to the CPU register.
The process of storing the data in the stack is called ‘pushing into’ the stack and the reverse process of transferring the data back from the stack to the CPU register is known as ‘popping off’ the stack. The stack is essentially Last-In-First-Out (LIFO) data segment. This means that the data which is pushed into the stack last will be on top of stack and will be popped off the stack first. The stack pointer is a 16-bit register that contains the offset address of the memory location in the stack segment.
The stack segment, like any other segment, may have a memory block of a maximum of 64 Kbytes locations, and thus may overlap with any other segments. Stack Segment register (SS) contains the base address of the stack segment in the memory.
The Stack Segment register (SS) and Stack pointer register (SP) together address the stack-top as explained below:
SS
5000H
SP
2050H
Physical address of Stack-top = 5000H * 10H + 2050H
If the stack top points to a memory location 52050H, it means that the location 52050H is already occupied with the previously pushed data. The next 16 bit push operation will decrement the stack pointer by two, so that it will point to the new stack-top 5204EH and the decremented contents of SP will be 204EH . This location will now be occupied by the recently pushed data.
Thus for a selected value of SS, the maximum value of SP=FFFFH and the segment can have maximum of 64K locations.
If the SP starts with an initial value of FFFFH, it will be decremented by two whenever a 16-bit data is pushed onto the stack. After successive push operations, when the stack pointer contains 0000H, any attempt to further push the data to the stack will result in stack overflow.
After a procedure is called using the CALL instruction, the IP is incremented to the next instruction. Then the contents of IP,CS and flag register are pushed automatically to the stack. The control is then transferred to the specified address in the CALL instruction i.e. starting address of the procedure. Then the procedure is executed.