讲解GA.2250、讲解Python设计程序

日期：2024-01-30 08:06

Programming Assignment #1 (Lab 1)
Class CSCI-GA.2250-001: Operating Systems – Spring24
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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 and assigning global addresses after placing the modules’ object code at global addresses.
Rather than dealing with complex x86 tool chains that I demonstrated in class, 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 core tasks of linkers such relocation, relative addressing,
symbol table maintenance and in particular error checking and reporting is handled in this notation as well.
The input to the linker is a file containing a sequence of tokens (symbols and integers and instruction mode characters).
Don’t assume tokens that make up a section to be on one line, nor 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 (M, A, R, I, 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 can be 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 (addrmode, instr), where addrmode is a
single character indicating the addressing mode as Module, Absolute, Relative, Immediate or External. instr is the
instruction (integer). Note that codecount defines the length/size of the module.
The instruction integer is comprised of an opcode (instruction/1000) and an operand (instruction%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:
(M) operand is the number of a valid module and is replaced with the base address of that module.
(A) operand is an absolute address which will never be changed in pass2; however it can’t be “>=” the machine size (512);
(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).
(I) an immediate operand is unchanged, but must be less than 900.
(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.
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 (1) the base address for each module and stores it in a module_base table, and (2) 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 are the module_base table the symbol table.
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.
While parsing you cannot store tokens, not between pass one and pass two, nor while parsing a single line. This practices
reentrant code which one encounters frequently in Operating Systems. This implies you can only parse and consume one
token at a time and can’t run through a full line and store tokens per line or file as indicated in the strtok() man page. To
enforce this we deduct 3 pts if you do this.
Programming Assignment #1 (Lab 1): Linker Professor Hubertus Franke
Class CSCI-GA.2250-001: Operating Systems – Spring24
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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 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 first definition of a symbol exceeds the size of the module, print a warning message in
pass1 after processing the module and treat the address given as 0 (relative to the module). If the symbol is a
redefinition then print a warning message (see message below).
6. If an external operand is too large to reference an entry in the use list, print an error message and treat the operand as
relative=0.
7. If a symbol appears in a use list but is not actually used in the module (i.e., not referred to in any E-type address),
print a warning message and continue. If the same unused symbol appears multiple times in uselist, multiple
warnings are to be printed.
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 operand (I) is encountered (i.e. >= 900), print an error and convert the operand value to 999.
11. If an illegal opcode is encountered (i.e. op >= 10), print an error and convert the