5-2 - Compilation and linking
*****************************
Compilation and link editing are the two main stages in the process
of translating a High Level Language (HLL) program to machine code
that can be executed on a CPU.
A small glossary
----------------
MACHINE INSTRUCTIONS Simple commands that the hardware can execute directly
GENERAL REGISTERS (or just REGISTERS) are small very fast memory units
built into the CPU, registers are used in almost all
CPU operations.
ADDRESSING MODES The methods in which a memory address can be specified
in a machine instruction.
PROGRAM COUNTER Special register keeping track of the memory location
of the next machine instruction to be executed.
CISC ARCHITECTURE Characterized by: full set of register-to-memory
instructions, variable length instructions, many
addressing modes, instructions typically decoded
and interpreted by a microprogram.
RISC ARCHITECTURE Characterized by: simple fixed-length instructions
(load/store), few addressing modes, instructions
typically decoded and executed directly by hardware.
ENTRY POINT Memory address where execution of a program/routine
starts.
CONTROL TRANSFER Stopping sequential execution of program code, and
jumping to a specified code location, performed by
changing the contents of the program counter.
JUMP Unconditional control transfer.
BRANCH Conditional control transfer.
MEMORY REFERENCE Accessing a specified place in memory that stores
some variable or machine instructions.
Machine instructions
--------------------
The CPU is an electronic machine, which constantly fetches
MACHINE INSTRUCTIONS from the main memory and executes them.
Machine instructions are simple commands that the hardware can
execute directly, typical (and basic) instructions:
1) Load a word from a certain location in main memory to a
certain GENERAL REGISTER
2) Perform some simple arithmetic operation (add, multiply
compare, etc) on data kept in a certain pair of registers
3) Store the contents of a certain register to a certain
location in main memory
4) Conditional jump (branch) or unconditional jump to a
pre-determined location in the program (this is really
just a memory location, as programs are loaded into
memory before execution).
Computer CPUs, either hardwired or using microcode are very limited,
every instruction (or micro-instruction) must be implemented by
dedicated digital logic electronic circuits, and so can't be too
complex.
Modern CPUs are usually designed according to the RISC (Reduced
Instruction Set Computer) paradigm, RISC CPUs can execute only a
small set of instructions, and so are simpler and faster. Of course
the RISC instructions are choosed so any needed operation can be
performed by a suitable sequence of them.
The older CISC paradigm (Complex Instruction Set Computers), takes
the opposite view, the CPU instruction set is made very large, every
instruction can be performed from every general register, in any
addressing mode (orthogonality), but even CISCs can execute only
relatively simple machine instructions.
A common opinion is that modern RISC CPUs are faster because it is
more efficient to build complex operations from smaller building
blocks, and the CISC wealth of instructions and modes is just slowing
the CPU and isn't useful to HLL compilers.
Other people maintain that RISCs are now faster than CISCs, not
because of the inherent superiority of their paradigm, but because
RISCs became fashionable and were built using modern technologies
(DEC NVAX chip is a modern fast CISC).
Addressing modes
----------------
Usually when we are loading/storing a register or jumping/branching
we need to specify a memory address, usually some (or all) of these
addressing modes are available:
Addressing mode Location of operand
----------------- ----------------------------------------------
Immediate/Literal In the machine instruction itself
Direct/Absolute Memory location whose address is specified in
the instruction
Indirect Memory location whose address is in a memory
location whose address is specified in the
machine instruction.
Register In a general register
Register-indirect Memory location whose address is in a register
whose name is specified in the instruction
Register-deferred Same as register-indirect
Autoincrement Register-indirect followed by an automatic increment
of the register contents by the operand size
Autodecrement Register-indirect preceded by an automatic decrement
of the register contents by the operand size
Displacement Memory location whose address is the sum of some
register's contents and a value specified in
the machine instruction
Relative Just like displacement mode, but the register
used is the program counter (with updated value)
Indexed Memory location whose address is the sum of some
register's contents and another register's
contents multiplied by a small constant that
equals a data item size
Indexed-indirect Memory location whose address is in the memory
location whose address is the sum of some
register's contents and another register's
contents multiplied by a small constant that
equals a data item size
Indexed-deferred Same as indexed-indirect
Remarks:
Displacement mode is sometimes called indexed mode, in that
case what we called indexed mode is probably unavailable.
The program counter is similar to other registers, but
changing its contents means performing a jump/branch.
The immediate/literal addressing mode is clearly not
suitable for memory variables.
FORTRAN programs and the underlying machine
-------------------------------------------
You can't really understand programming and how compilers works,
without examining the way the HLL source code is translated into
machine code.
Most compilers offer an option to generate an assembly listing of
the translated program, this is often the only way to check and
study how the compiler handles subtle points, e.g. if it does
shortcut-evaluations, handles properly DO-loop termination etc.
A small example program:
PROGRAM EXMPL
INTEGER INT
READ (*,*) INT
INT = INT + 1
WRITE (*,*) INT
END
And this is the translation to the classical assembly
language of the VAX, with a little editing to make
it more readable:
.TITLE EXMPL
.IDENT 01
0000 .PSECT $CODE
! PROGRAM EXMPL
0000 EXMPL::
0000 .WORD ^M
0002 MOVAL $LOCAL, R11
! READ (*,*) INT
0009 MNEGL #4, -(SP)
000C CALLS #1, FOR$READ_SL
0013 PUSHAL INT(R11)
0015 CALLS #1, FOR$IO_L_R
001C CALLS #0, FOR$IO_END
! INT = INT + 1
0023 ADDL3 #1, INT(R11), R12
! WRITE (*,*) INT
0027 MNEGL #1, -(SP)
002A CALLS #1, FOR$WRITE_SL
0031 PUSHL INT%R12
0033 CALLS #1, FOR$IO_L_V
003A CALLS #0, FOR$IO_END
! END
0041 MOVL #1, R0
0044 RET
.END
Symbolic memory locations
-------------------------
HLL programmers don't have to work directly with all these bewildering
addressing modes, the compiler uses them in a user-transparent way to
create the HLL language constructs.
HLLs (and even assembly) implements the following 'code mechanisms':
1) Explicit/Implicit statement labels, used as targets
for control transfer.
2) Memory variables - memory locations of suitable size,
that can be referenced by symbolic names instead of
memory addresses.
The most direct way to handle statement labels and memory variables
is to compute and keep the offsets (e.g. from the program beginning)
of the relevant machine instructions while translating the HLL to
machine code.
The two 'mechanisms' are actually implemented using suitable addressing
modes, and the above-mentioned 'offset bookeeping'.
The relocation problem
----------------------
A program usually contains many references to memory locations,
either as operands or as targets for jumps/branches.
It is convenient to have the compiler generate machine code that
can be easily combined with other pieces of machine code and form
one program, it is also convenient that a program will be able to
run no matter where in the main memory it was loaded.
What we are looking for is a way to write machine code that don't
use explicit memory addresses, that way the code will be able to
execute no matter where in memory it was loaded. That property is
called RELOCATABILITY.
To achieve that, all memory references contained in the program
must be either invariant to the memory location in which they will
be loaded, or some kind of an address translation process must
take place.
Possible methods to achieve location invariance are referring to
memory variables using relative mode, and using displacement mode
with some register (called base register) pointing to the beginning
of the loaded program.
The relative mode method works because when an instruction starts
executing the program counter points to it, if we make memory
references relative to the program counter they will be independant
of the place where the program was loaded.
In the displacement mode method we just have to load the base register
with the address of the loaded program beginning.
The relocation problem between modules
--------------------------------------
Using the relative/displacement addressing modes solves most of the
relocation problems in a single compilation unit.
In FORTRAN ....
There are some cases we will have to translate special memory references
by adding to them the memory address of the beginning of the program.
Symbol resolution
-----------------
Programs starts execution when the program counter is loaded with the
address of the main entry point,
A much more important factor in the social movement than those already mentioned was the ever-increasing influence of women. This probably stood at the lowest point to which it has ever fallen, during the classic age of Greek life and thought. In the history of Thucydides, so far as it forms a connected series of events, four times only during a period of nearly seventy years does a woman cross the scene. In each instance her apparition only lasts for a moment. In three of the four instances she is a queen or a princess, and belongs either to the half-barbarous kingdoms of northern Hellas or to wholly barbarous Thrace. In the one remaining instance208— that of the woman who helps some of the trapped Thebans to make their escape from Plataea—while her deed of mercy will live for ever, her name is for ever lost.319 But no sooner did philosophy abandon physics for ethics and religion than the importance of those subjects to women was perceived, first by Socrates, and after him by Xenophon and Plato. Women are said to have attended Plato’s lectures disguised as men. Women formed part of the circle which gathered round Epicurus in his suburban retreat. Others aspired not only to learn but to teach. Arêtê, the daughter of Aristippus, handed on the Cyrenaic doctrine to her son, the younger Aristippus. Hipparchia, the wife of Crates the Cynic, earned a place among the representatives of his school. But all these were exceptions; some of them belonged to the class of Hetaerae; and philosophy, although it might address itself to them, remained unaffected by their influence. The case was widely different in Rome, where women were far more highly honoured than in Greece;320 and even if the prominent part assigned to them in the legendary history of the city be a proof, among others, of its untrustworthiness, still that such stories should be thought worth inventing and preserving is an indirect proof of the extent to which feminine influence prevailed. With the loss of political liberty, their importance, as always happens at such a conjuncture, was considerably increased. Under a personal government there is far more scope for intrigue than where law is king; and as intriguers women are at least the209 equals of men. Moreover, they profited fully by the levelling tendencies of the age. One great service of the imperial jurisconsults was to remove some of the disabilities under which women formerly suffered. According to the old law, they were placed under male guardianship through their whole life, but this restraint was first reduced to a legal fiction by compelling the guardian to do what they wished, and at last it was entirely abolished. Their powers both of inheritance and bequest were extended; they frequently possessed immense wealth; and their wealth was sometimes expended for purposes of public munificence. Their social freedom seems to have been unlimited, and they formed combinations among themselves which probably served to increase their general influence.321 The old religions of Greece and Italy were essentially oracular. While inculcating the existence of supernatural beings, and prescribing the modes according to which such beings were to be worshipped, they paid most attention to the interpretation of the signs by which either future events in general, or the consequences of particular actions, were supposed to be divinely revealed. Of these intimations, some were given to the whole world, so that he who ran might read, others were reserved for certain favoured localities, and only communicated through the appointed ministers of the god. The Delphic oracle in particular enjoyed an enormous reputation both among Greeks and barbarians for guidance afforded under the latter conditions; and during a considerable period it may even be said to have directed the course of Hellenic civilisation. It was also under this form that supernatural religion suffered most injury from the great intellectual movement which followed the Persian wars. Men who had learned to study the constant sequences of Nature for themselves, and to shape their conduct according to fixed principles of prudence or of justice, either thought it irreverent to trouble the god about questions on which they were competent to form an opinion for themselves, or did not choose to place a well-considered scheme at the mercy of his possibly interested responses. That such a revolution occurred about the middle of the fifth century B.C., seems proved by the great change of tone in reference to this subject which one perceives on passing from Aeschylus to Sophocles. That anyone should question the veracity of an oracle is a supposition which never crosses the mind of the elder dramatist. A knowledge of augury counts among the greatest benefits222 conferred by Prometheus on mankind, and the Titan brings Zeus himself to terms by his acquaintance with the secrets of destiny. Sophocles, on the other hand, evidently has to deal with a sceptical generation, despising prophecies and needing to be warned of the fearful consequences brought about by neglecting their injunctions. The stranger had a pleasant, round face, with eyes that twinkled in spite of the creases around them that showed worry. No wonder he was worried, Sandy thought: having deserted the craft they had foiled in its attempt to get the gems, the man had returned from some short foray to discover his craft replaced by another. “Thanks,” Dick retorted, without smiling. When they reached him, in the dying glow of the flashlight Dick trained on a body lying in a heap, they identified the man who had been warned by his gypsy fortune teller to “look out for a hidden enemy.” He was lying at full length in the mould and leaves. "But that is sport," she answered carelessly. On the retirement of Townshend, Walpole reigned supreme and without a rival in the Cabinet. Henry Pelham was made Secretary at War; Compton Earl of Wilmington Privy Seal. He left foreign affairs chiefly to Stanhope, now Lord Harrington, and to the Duke of Newcastle, impressing on them by all means to avoid quarrels with foreign Powers, and maintain the blessings of peace. With all the faults of Walpole, this was the praise of his political system, which system, on the meeting of Parliament in the spring of 1731, was violently attacked by Wyndham and Pulteney, on the plea that we were making ruinous treaties, and sacrificing British interests, in order to benefit Hanover, the eternal millstone round the neck of England. Pulteney and Bolingbroke carried the same attack into the pages of The Craftsman, but they failed to move Walpole, or to shake his power. The English Government, instead of treating Wilkes with a dignified indifference, was weak enough to show how deeply it was touched by him, dismissed him from his commission of Colonel of the Buckinghamshire Militia, and treated Lord Temple as an abettor of his, by depriving him of the Lord-Lieutenancy of the same county, and striking his name from the list of Privy Councillors, giving the Lord-Lieutenancy to Dashwood, now Lord Le Despencer. "I tell you what I'll do," said the Deacon, after a little consideration. "I feel as if both Si and you kin stand a little more'n you had yesterday. I'll cook two to-day. We'll send a big cupful over to Capt. McGillicuddy. That'll leave us two for to-morrer. After that we'll have to trust to Providence." "Indeed you won't," said the Surgeon decisively. "You'll go straight home, and stay there until you are well. You won't be fit for duty for at least a month yet, if then. If you went out into camp now you would have a relapse, and be dead inside of a week. The country between here and Chattanooga is dotted with the graves of men who have been sent back to the front too soon." "Adone do wud that—though you sound more as if you wur in a black temper wud me than as if you pitied me." "Wot about this gal he's married?" "Don't come any further." "Davy, it 'ud be cruel of us to go and leave him." "Insolent priest!" interrupted De Boteler, "do you dare to justify what you have done? Now, by my faith, if you had with proper humility acknowledged your fault and sued for pardon—pardon you should have had. But now, you leave this castle instantly. I will teach you that De Boteler will yet be master of his own house, and his own vassals. And here I swear (and the baron of Sudley uttered an imprecation) that, for your meddling knavery, no priest or monk shall ever again abide here. If the varlets want to shrieve, they can go to the Abbey; and if they want to hear mass, a priest can come from Winchcombe. But never shall another of your meddling fraternity abide at Sudley while Roland de Boteler is its lord." "My lord," said Edith, in her defence, "this woman has sworn falsely. The medicine I gave was a sovereign remedy, if given as I ordered. Ten drops would have saved the child's life; but the contents of the phial destroyed it. The words I uttered were prayers for the life of the child. My children, and all who know me, can bear witness that I have a custom of asking His blessing upon all I take in hand. I raised my eyes towards heaven, and muttered words; but, my lord, they were words of prayer—and I looked up as I prayed, to the footstool of the Lord. But it is in vain to contend: the malice of the wicked will triumph, and Edith Holgrave, who even in thought never harmed one of God's creatures, must be sacrificed to cover the guilt, or hide the thoughtlessness of another." "Aye, Sir Treasurer, thou hast reason to sink thy head! Thy odious poll-tax has mingled vengeance—nay, blood—with the cry of the bond." HoME古一级毛片免费观看
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