CSCI-GA.2250-001/002-Lab 1 Linker Solved

In this lab you will be implementing a two-pass linker. In general, a linker takes individually compiled code/object modules and creates a single executable by resolving external symbol references (e.g. variables and functions) and module relative addressing by assigning global addresses after placing the modules’ object code at global addresses.  

 

Rather than dealing with complex x86 tool chains, we assume a target machine with the following properties: (a) word addressable, (b) addressable memory of 512 words, and (c) each valid word is represented by an integer (<10000). [ I know that is a really strange machine, but I once saw an UFO too].

 

The input to the linker is a file containing a sequence of tokens (symbols and integers and instruction type characters). Don’t assume tokens that make up a section to be on one line, don’t make assumptions about how much space separates tokens or that lines are non-empty for that matter or that each input conforms syntactically. Symbols always begin with alpha characters followed by optional alphanumerical characters, i.e.[a-Z][a-Z0-9]*. Valid symbols can be up to 16 characters. Integers are decimal based. Instruction type characters are (I, A, R, E). Token delimiters are ‘ ‘, ‘\t’ or ‘\n’.

 

The input file to the linker is structured as a series of “object module” definitions.

Each “object module” definition contains three parts (in fixed order): definition list, use list, and program text.  

 

•       definition list consists of a count defcount followed by defcount pairs (S, R) where S is the symbol being defined and R is the relative word address (offset) to which the symbol refers in the module (0-based counting).  

•       use list consists of a count usecount followed by usecount symbols that are referred to in this module. These could include symbols defined in the definition list of any module (prior or subsequent or not at all).

•       program text consists of a count codecount followed by codecount pairs (type, instr), where type is a single character indicating the addressing mode as Relative, External. Immediate or Absolute and instr is the instruction

(integer)  Note that codecount defines the length of the module.  

 

An instruction is composed of an integer that is comprised of an opcode (op/1000) and an operand (op mod 1000). The opcode always remains unchanged by the linker. For the instruction value read an integer and ensure opcode < 10, see errorcodes below. The operand is modified/retained based on the instruction type in the program text as follows:

(R) operand is a relative address in the module which is relocated by replacing the relative address with the absolute address of that relative address after the module’s global address has been determined (absolute_addr = module_base+relative_addr). (E) operand is an external address which is represented as an index into the uselist. For example, a reference in the program text with operand K represents the Kth symbol in the use list, using 0-based counting, e.g., if the use list is ‘‘2 f g’’, then an instruction ‘‘E 7000’’ refers to f, and an instruction ‘‘E 5001’’ refers to g. You must identify to which global address the symbol is assigned and then replace the operand with that global address. (I) an immediate operand is unchanged.

(A) operand is an absolute address which will never be changed in pass2; however it can’t be “=” the machine size (512);

 

 

The linker must process the input twice (that is why it is called two-pass) (to preempt the favored question: “Can I do it in one pass?” → NO, because storing tokens makes your program more complex). Pass One parses the input and verifies the correct syntax and determines the base address for each module and the absolute address for each defined symbol, storing the latter in a symbol table. The first module has base address zero; the base address for module X+1 is equal to the base address of module X plus the length of module X (defined as the number of instructions in a module). The absolute address for symbol S defined in module M is the base address of M plus the relative address of S within M. After pass one print the symbol table (including errors related to it (see rule2 later)). Do not store parsed tokens, the only data you should and need to store between passes is the symboltable.

Pass Two again parses the input and uses the base addresses and the symbol table entries created in pass one to generate the actual output by relocating relative addresses and resolving external references. You should reuse pass-1 parser code just with different actions. You must clearly mark your two passes in the code through comments and/or proper function naming.

 

Other requirements: error detection, limits, and space used. 

 

To receive full credit, you must check the input for various errors (test inputs will have lots of errors). All errors/warnings should follow the message catalog provided below. We will do a textual difference against a reference implementation to grade your program. Any reported difference will indicate a non-compliance with the instructions provided and is reported as an error and results in deductions.

You should continue processing after encountering an error/warning (other than a syntax error) and you should be able to detect multiple errors in the same run.

1.       You should stop processing if a syntax error is detected in the input, print a syntax error message with the line number and the character offset in the input file where observed. A syntax error is defined as a missing token (e.g. 4 used symbols are defined but only 3 are given) or an unexpected token. Stop processing and exit.

2.       If a symbol is defined multiple times, print an error message and use the value given in the first definition. The error message is to appear as part of printing the symbol table (following symbol=value printout on the same line).

3.       If a symbol is used in an E-instruction but not defined anywhere, print an error message and use the value absolute zero.

4.       If a symbol is defined but not used, print a warning message and continue.

5.       If an address appearing in a definition exceeds the size of the module, print a warning message and treat the address given as 0 (relative to the module).

6.       If an external address is too large to reference an entry in the use list, print an error message and treat the address as immediate.

7.       If a symbol appears in a use list but is not actually used in the module (i.e., not referred to in an E-type address), print a warning message and continue.

8.       If an absolute address exceeds the size of the machine, print an error message and use the absolute value zero.

9.       If a relative address exceeds the size of the module, print an error message and use the module relative value zero (that means you still need to remap “0” that to the correct absolute address).

10.    If an illegal immediate value (I) is encountered (i.e. = 10000), print an error and convert the value to 9999.  

11.    If an illegal opcode is encountered (i.e. op = 10), print an error and convert the <opcode,operand to 9999.

 

The following exact limits are in place.

a)       Accepted symbols should be upto 16 characters long (not including terminations e.g. ‘\0’), any longer symbol names are erroneous.

b)       a uselist or deflist should support 16 definitions, but not more and an error should be raised.

c)       number of instructions are unlimited (hence the two pass system), but in reality they are limited to the machine size.

d)       Symbol table should support at least 256 symbols (reference program supports exactly 256 symbols).

 

There are several sample inputs and outputs provided as part of the sample input files / output files (see NYU Classes). The first (input-1) is shown below and the second (input-2) is a re-formatted version of the first. They both produce the same output as the input is token-based and hence present the same content to the linker. Many of the input sets contain errors that you are to detect as described above. Note that when you have questions regarding errors, please first make sure the structure of the input is not messing with your mind. We will run your lab on these (and other) input sets.

 

1  xy 2 

2  z xy 

5         R 1004  I 5678  E 2000  R 8002  E 7001 0 

1 z 

6         R 8001  E 1000  E 1000  E 3000  R 1002  A 1010 0 

1  z 

2  R 5001  E 4000 

1  z 2 

2  xy z 

3  A 8000  E 1001  E 2000 

                           

Your output is expected to strictly follow this format (with exception of empty lines):  

 

Symbol Table xy=2 z=15 

 

Memory Map 

000: 1004 

001: 5678 

002: 2015 

003: 8002 

004: 7002 

005: 8006 

006: 1015 

007: 1015 

008: 3015 

009: 1007 

010: 1010 

011: 5012 

012: 4015 

013: 8000 

014: 1015 

015: 2002 

 

The following output is heavily annotated for clarity and class discussion. You are not creating this output nor do we have means to check it. However, it should help you understand the operation and mapping of symbols etc.

 

Symbol Table xy=2 z=15 Memory Map 

+0 

0:      R 1004        1004+0 =  1004 

1:      I 5678                  5678 

2: xy:  E 2000 -z              2015 

3:      R 8002        8002+0 =  8002 

4:      E 7001 -xy             7002 +5 

0:      R 8001        8001+5 =  8006 

1:      E 1000 -z              1015 

2:      E 1000 -z              1015 

3:      E 3000 -z              3015 

4:      R 1002        1002+5 =  1007 

5:      A 1010                  1010 +11 

0:      R 5001        5001+11=  5012 

1:      E 4000 -z              4015 +13 

0:      A 8000                  8000 

1:      E 1001 -z              1015 

2 z:    E 2000 -xy             2002 

 

Note that even an empty program should have the “Symbol Table” and “Memory Map” line.

 

For a test case to pass you must catch ALL warning/errors and generate the correct output for a given input file.

 

Example:   

 

Symbol Table 

X21=3 

X31=4 

 

Memory Map 

000: 1003 

001: 1003 

002: 1003 

003: 2000  Error: Absolute address exceeds machine size; zero used 004: 3000  Error: Relative address exceeds module size; zero used 

 

Warning: Module 3: X31 was defined but never used 

 

Parse errors should abort processing.

Error messages must be following the instruction as shown above.

Warnings message locations are defined further down. Module counting starts at 1.

 

I provide in C the code to print parse errors, which also gives you an indication what is considered a parse error.

 

void __parseerror(int errcode) {     static char* errstr[] = { 
 
        "NUM_EXPECTED",        
// Number expect 
        "SYM_EXPECTED",        
// Symbol Expected 
        "ADDR_EXPECTED",       
// Addressing Expected which is A/E/I/R   
        "SYM_TOO_LONG",       
// Symbol Name is too long 
        "TOO_MANY_DEF_IN_MODULE", 
// 16  
        "TOO_MANY_USE_IN_MODULE",     // 16 

        "TOO_MANY_INSTR”,             // total num_instr exceeds memory size (512)   

    }; 

    printf("Parse Error line %d offset %d: %s\n", linenum, lineoffset, errstr[errcode]); }  

(Note: line numbers start with 1 and offsets in the line start with 1, offsets should indicate the first character offset of the token that is wrong, not the last). Tabs (‘\t’) count as one character.

Error messages have the following text and should appear right at the end of the line you are printing out

 

"Error: Absolute address exceeds machine size; zero used"                                                       (see rule 8) 

                "Error: Relative address exceeds module size; zero used"                                                          (see rule 9) 

                "Error: External address exceeds length of uselist; treated as immediate"                                 (see rule 6) 

                "Error: %s is not defined; zero used"                         (insert the symbol name for %s)              (see rule 3) 

                "Error: This variable is multiple times defined; first value used"                                               (see rule 2) 

                "Error: Illegal immediate value; treated as 9999"                                                                       (see rule 10) 

                "Error: Illegal opcode; treated as 9999"                                                                                      (see rule 11) 

 

Warning messages have the following text and are on a separate line.  

 

                "Warning: Module %d: %s too big %d (max=%d) assume zero relative\n"                                (see rule 5)  

                "Warning: Module %d: %s appeared in the uselist but was not actually used\n"                        (see rule 7)   "Warning: Module %d: %s was defined but never used\n”                                                          (see rule 4) 

 

Locations for these warnings are: 

Rule 5: to be printed after each module in pass1 

Rule 7: to be printed after each module in pass2 (so actually interspersed in the memory map (see  out-9) Rule 4: to be printed after pass 2 (i.e. all modules have been processed)   ( see out-3). 

 

Parse Error Location: 

 

Parse errors are to be located at the first character of the wrong token, or if end-of-file is reached at the end of file location. There is one special case when the eof ends with `\n’. My expectation is that the line number reported actually exists in the file and that an editor (e.g. vi) can jump to it. In this particular case the linenumber to be reported is the last line read and the last position of that line, not the next line and offset 1 (see input-12  for an example). The error is at the very last position of the line. Reason is when one does a linecount on the file  (“wc –l input-12”)  it shows 3 not 4.

 

Hint: for each parser error and warning error mentioned above you should have code checking for that.