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A Linker for SIC/XE Hypothetical Computer
ˇ
Kisek, Nejc
Abstract: The article describes the de- multiple object code sections together; loader,
sign and implementation of a linker for the that inserts the object code in the correct part
SIC/XE hypothetical computer, described in of computer’s memory; compiler, that converts
Leland Beck’s book System Software. Linker a high level programming language into assem-
was developed as a part of SicTools collection bly code and an operating system, that con-
consisting of system software tools for the same trols resources, communicates with the hard-
hypothetical computer. We start by introduc- ware and overlooks user applications.
ing SIC/XE and existing tools for working with SicTools [6] is a collection of system soft-
it, followed by a few concepts behind the link-
ing process. Then we describe the implemented ware utilities for SIC/XE that includes an as-
linker, list its functionalities and describe how sembler, a simulator and a linker. It was devel-
to use it. We focus on using the linker as a li- oped for enhancing practical demonstrations in
brary in another program, but we also give some a system software course at University of Ljubl-
instructions on using it on its own. Finally, to jana’s Faculty of Computer and Information sci-
better illustrate the process of linking, we also ence. It is used at several faculties around the
show a simple example. world, mainly at courses following the Beck’s
book. The included assembler allows us to
Index Terms: linker, SIC/XE, SicTools, sys- translate assembly code into object files, fol-
tem software lowing specifications from the System Software
book. The simulator features a graphical inter-
1. Introduction face, an overview of CPU registers and entire
memory. It can run at different frequencies and
IC/XE (Simplified Instructional Computer supports breakpoints or step by step execution.
Swith Extra Equipment) is a hypothetical Additionally, it can simulate a graphical or a
processor from Leland Beck’s textbook System textual screen and can interact with standard
Software [1]. The book uses it to show how as- input/output or external files.
semblers, linkers, loaders, compilers and other The linker utility was added to SicTools as
parts of system software work. The main pur- part of a bachelor’s degree thesis [4] following
poseofthisprocessor is to have a minimal set of the specifications from Beck’s book [1]. We
features that most real processors have, without also used concepts from [5] and followed the
extra functionalities or simplifications. Because approach to generation of ideas as documented
of that, concepts for working with SIC/XE are in [7].
also useful when working with real processors. In the following section we describe the
System software is software that supports main concepts behind linking. In Section 3 we
the use and operation of other programs and list the features of our linker implementation.
the computer itself. A few examples of such Then we present an overview of how each part
programs are assembler, that converts assembly of the library can be used in Section 4 and how
language into object code; linker, that connects the linker can be used as a standalone program
in Section 5. In Section 6 we show a simple ex-
Manuscript received Jul, 2017 and accepted for the ample of linking and then conclude with some
journal in Nov, 2017. ideas for future work.
Nejc Kisek, Faculty of Computer and Informa-
ˇ
tion Science, University of Ljubljana, Slovenia (e-mail:
nk4741@student.uni-lj.si).
2. Linking contain the code itself; M Records that describe
Linking is a process of combining multiple ob- how addresses in the code should be modified
ject files to create the whole program. Object during linking or loading; D Records that define
files are comprised of multiple control sections, which symbols in the section should be exter-
which are independent parts of the program, nal; R Records that refer to symbols from other
but can be connected with a system of refer- sections and a final E Record marking the end
ences. A symbol (variable) inside one section of a section.
can be seen by other sections if we define it as During the linking, D and R records tell the
an external symbol. Other sections can refer to linker which sections contain external symbol
symbols defined in this way and use them. In references and their definitions, while M records
Figure 1 we can see how a program in multiple specify which addresses in the code belong to
assembly files is processed before we can run it. a reference and should be modified.
3. Implemented Linker
The main functionality of the implemented
linker is linking multiple object files into a sin-
gle one and resolving any references that ex-
ist between them. In a typical scenario, we
have several object files comprised of multiple
control sections with references between them
as an input to the liker, which then outputs
a single .obj file containing only one control
section with all references resolved. If this is
not exactly what we want to achieve, the im-
Figure 1: Linking process plemented linker has a few extra options that
change how the linking process works.
During the process, linker first assigns an Partial linking. This option can be used,
address to each section and checks which sec- when we want to link multiple control sections
tions contain externally defined symbols. After where not all references can be resolved – some
that, it parses all the sections and replaces any of them may refer to definitions that are not
references with actual addresses from previous present in any control section. If we enable
step. Finally, linker combines all sections into partial linking, the linker output will still be just
one and writes it to a file. one object file with one control section, but it
Thereareafewdetailsthatwedidnotmen- will also include refer and modify records for
tion, for example that addresses in SIC/XE can each reference without a define record in any
be absolute or relative. Absolute addresses re- of the output files. This is useful, when we do
fer to a specific point in memory and is not not have all the object files of a program, but
supported in our linker implementation, since we want to link just the ones we already have.
sections can not be moved to an arbitrary place
in the memory. Relative addresses are adjusted Preserving external definitions. This is an
by the loader just before the program is run, option that keeps the define records in the out-
based on where in memory the code was placed. put object file after resolving the references. It
Linker handles different types of relative ad- can be used for debugging, but we might also
dresses in a slightly different way, to ensure the use it for further linking the object file.
loader will still understand them after they are Let us demonstrate with an example. We
in the same section. would like to link object files containing refer-
SIC/XE object files contain different types ences A, B and C, but we only have files with
of records: H Record that contains name ad- definitions of A and B available and have to use
dress and length of the section; T Records that
partial linking option to skip C. When we get puts the section to a text file and returns the
the object file for C it might include its own File class pointing to it.
references to A and B. If we want to link it
with our partially linked program, we will have
to include the files for A and B again, which
will lead to duplicated code. The preserving ex-
ternal definitions option during the first partial
linking allows the original A and B to be used
in the second linking as well, without relinking
the whole program or duplicating code.
Modifying Sections. Reordering and modi-
fying sections can be done in two ways. Usually
wejustwanttospecifythesectionthatneedsto
befirst(wheretheprogramshouldstart), which
can be done using the main section option. Figure 2: Linker classes and methods
When more modifications are needed, we can
also change the order of other sections manu-
ally as well as rename or delete them. This can Specifying Options. We can control the
be done programatically when using the linker linking process with variables inside the
library or with the commandline or graphical Options class. Most of them we mentioned in
interface included in SicTools. the previous chapter: attributes force, keep
and verbose of boolean type represent the
Verbose Mode Thisisanoptionusedforde- partial linking, preserving definitions and ver-
bugging and prints all actions that happen dur- bose mode options, respectively, while the at-
ing the linking to standard output. tribute main of type String specifies the name
of the section that should be first in the re-
4. Linker Library sulting output. In addition to that, we have
outputPath and outputName that are used by
Thelinker is implemented as a Java library that the Writer utility and represent the full path to
can be used via the Linker class. Files, classes, and the name of the output file. There are also
methods and operations involved in linking are graphical and editing attributes of boolean
illustrated on the diagram in Figure 2. type that are used by the included linker tool.
The Linker constructor receives a list of Modifying Sections. If we want to change
filenames for the object files we want to link the order of sections, modify them or change
and an Options class that defines any addi- the references used in them, we can perform
tional options mentioned in the previous chap- the linking in smaller separate steps instead of
ter. If we do not need to reorder any sec- using the link method.
tions, we can just call the link method, which After creating the Linker object, we call
will read all specified files and link the sec- its parse method, which will read the input
tions. The method will return the result in the files and return a Sections object. It contains
form of a single Section class. Section class the name of the whole program and the list of
stores information about the control section it- Section objects, each representing one control
self (name, start address and length) and all section that was found in the input files. The
the records that the section contains. order of sections in the list represents how they
A section can be written to an object file, will be ordered in the linked program.
with the Writer class. Its constructor takes Sections class also has methods for mod-
Section and Options classes as arguments ifying containing sections, references or defini-
and implements a method write, which out- tions: rename, move or remove for sections
and renameRef, renameDef or removeRef, there are checkboxes and input fields. Object
removeDef for references. When renaming we files are added via the Add .obj button and
need to make sure all names are at most six are displayed on the central list, where they can
characters long, that section names are unique also be reordered.
and that definition names are unique within
each section. Renaming a reference with an
already used name will combine them into one
(their M records will use the same symbol).
We can finalise the linking with the
passAndCombine method in the Linker
class. It accepts the modified Sections as an
argument, performs the linking and outputs a
single Section object. The result is a finished Figure 3: The graphical standalone linker
linked section, similar to the result of the link
method, and can be written to a file with the The tool includes a section editor (graphi-
Writer utility in the same way as mentioned cal or in a terminal), which is launched if user
in the beginning of this chapter. In fact, the selected the -e option and allows user to move,
link method internally just calls parse and rename or remove sections, definitions or refer-
passAndCombine one after another, without ences using the Sections methods, mentioned
modifying the Sections object. in the previous chapter.
5. Using the Standalone Linker 6. Example
Linker can be launched from the menu in the Complete code for the program used in the fol-
SicTools simulator or as a standalone program lowing example is available in the SicDemos
with java -cp sictools.jar sic.Link fol- GitHub repository [2] under the name Link -
lowed by option flags and paths to input files. Factorial. The example links five object files
Available flags: into one program that calculates the factorial
of numbers 1 to 10. Each part of the pro-
• -h displays instructions, gram is written in a separate file to better
• -o followed by a filename specifies the demonstrate the linking process: stack.obj
output file, contains routines for working with the stack,
print.objcontains the code for displaying the
• -g launches the graphical version of the result, fact.obj contains the factorial func-
standalone linker, tion, main.obj connects all the parts together
andending.objmarkstheendoftheprogram.
• -f, -k and -v enable partial linking, pre-
serving external definitions and verbose Hmain 0000000000F3
mode options respectively, R end fact print resultstinit
T0000001E011 00000 4B1 00000 0100010F1 00000
• -m followed by a section name specifies T00001E154B1 00000 031 00000 4B1 00000 3F2FDB
which section should be first in the linked M 000001 05+ end
program, M 000005 05+ stinit
• -e will launch the section editor after M 00000C 05+ result
reading the input files. M 00001F 05+ fact
M 000023 05+ result
Thegraphical interface is shown in Figure 3 M 000027 05+ print
and has the same options presented in a slightly E000000
different way – instead of commandline flags Figure 4: Part of the object file main.obj
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