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Systems Programming

Systems Programming. Chapter 2 Assembler IV. 2.2.2 Program Relocation. More than one program at a time sharing the memory and other resource of the machine. The program in Figure 2.1 is an example of an absolute program. This program has to be loaded at address 1000.

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Systems Programming

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  1. Systems Programming Chapter 2 Assembler IV CPS4200 System Programming 2007 Spring

  2. 2.2.2 Program Relocation • More than one program at a time sharing the memory and other resource of the machine. • The program in Figure 2.1 is an example of an absolute program. This program has to be loaded at address 1000. • Since the assembler does not know the actual location where the program will be loaded, it cannot make the necessary changes in the addresses used by the program. • However, the assembler can identify for the loader those parts of the program that need modification. • An object program that contains the information necessary to perform this kind of modification is called a re-locatable program. CPS4200 System Programming 2007 Spring

  3. 2.2.2 Program Relocation • See Figure 2.7 for the example of program relocation. • The JSUB instruction from line 15 is loaded at address 0006 and the address field contains 01036 which is the address of the instruction labeled RDREC. See Figure 2.7(a), 2.7(b) and 2.7(c) for different starting location. • However, no matter where the program is loaded, RDREC is always 1036 bytes past the starting address of the program. CPS4200 System Programming 2007 Spring

  4. 2.2.2 Program Relocation • We can solve the relocation problem in the following way: • When the assembler generates the object code for the JSUB instruction we are considering, it will insert the address RDREC relative to the start of the program. • The assembler will also produce a command for the loader, instructing it to add the beginning address of the program to the address field in the JSUB instruction at load time. CPS4200 System Programming 2007 Spring

  5. 2.2.2 Program Relocation • The command for the loader must also be a part of the object program. A Modification record having the following format: • 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-byte. CPS4200 System Programming 2007 Spring

  6. 2.2.2 Program Relocation • The length is stored in half-bytes because the address to be modified may not occupy an integral number of bytes. For example JSUB instruction occupied 20 bits which is 5 half-bytes. • The starting location is the location of the byte containing the leftmost bits of the address field to be modified. If this field occupies an odd number of half-bytes, it is assumed to begin the middle of the first byte at the starting location. CPS4200 System Programming 2007 Spring

  7. 2.2.2 Program Relocation • For the JSUB instruction, the Modification record would be M00000705. • It specifies that the beginning address of the program is to be added to a field that begins at 000007 (relative to the start of the program) and is 5 half-bytes in length. • 4B101036, the first 12 bits (4B1) will remain unchanged. The program load address will be added to the last 20 bits (01036) to produce the correct operand address. CPS4200 System Programming 2007 Spring

  8. 2.2.2 Program Relocation • Only Lines 35 and 65 in Figure 2.6 must be performed the same kind of relocation. This is because: • Either instructions operand is not a direct memory address; or • The operand is specified using program-counter relative or relative addressing. • For example, line 10 (STL RETADR) is assembled using program counter relative addressing with displacement 02D. No matter where the program is loaded in memory, the word labeled RETADR will always be 2D bytes away from the STL instruction. CPS4200 System Programming 2007 Spring

  9. 2.2.2 Program Relocation • The distance between LENGTH and BUFFER will always 3 bytes; thus the displacement in the base relative instruction on line 160 will be correct without modification. This is because the content of the base register will, of course, depend upon where the program-counter relative instruction LDB #LENGTH is executed. • Therefore the only such direct addresses are found in extended format (4-byte). CPS4200 System Programming 2007 Spring

  10. 2.2.2 Program Relocation • In Figure 2.8, the starting address of 0 in text records are interpreted as relative, rather than absolute, locations. • There is one Modification record for each address field that needs to be changed when the program is relocated . CPS4200 System Programming 2007 Spring

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