Virtual_segments Virtual Segments

virtual_segments virtual_segments

User Manual: virtual_segments

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Vlrtu6. Ser,nenta Article

Since I wrote the S/W for Version I.S, I have dlacovered how to get It
wo r kin 9 0 n Ve r II ('0 n I I I • 0 (9 n d I V • n Iss Im I I art 0 I I I • n I nth I s r e q a r d ) •
On the Mlcroenglne version IV.O there are no lIer,nent tahles to load, so I
played wIth the Seoment InformAtion Alocks (SIRs). These are described In
the IV.O manual on page 34, and the format for the Mlcroenqlne Is qlven
bel OWl

= record
Seqbase I
Seqlenq I
xxx
Segaddr r
SeQunlt
Segref
SeQSP
end,

I work for a companv (Energy Data Systems) which has heen trylnQ to do some
fairly complex applicatIons
In UCSD Pascal, and we ran Into the ·se~nt
barrier' In the spring of this year.
We considered modlfylnq
the lCSr>
operating system, since we had source for It, hut decided to try to sLick
to solutions which required as few modifications to operatlnQ syst~ code
8S possible.
After s~ thouQht
I c~e up with an Interim meRna of
ame I lor a tin 9 the pro b I em, wh I chI . w I I Ide &c rib e I nth I SAt tic Ie.

518

"Integeq
integer;
Inteqer;
Intefler;
Inteqer,
Inteqer;
Integer,

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There· Is a p-machlne regIster on the Mlcroenqlne which III a pointer to
9n "arrayrseqranqel of ·SIA", and the value of thIs register may be loaded
Into the variable "ptr" with

}~..

where LPR = 157 and 5TO = 196. One can then mess with the SIA for
seqnent 'I' with ptr·rll.fleldn~e, and this Is how I mede Virtual Segments
work on the Mlcroenglne; I ass~e that ~klng It work under IV.O would be
s Imll sr •

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Un for tun 8 tel y, u:(
Pas cal a I I ow s the pro q r emne r a c c e s s ·t 0 0 n I y s I x S f! (fne n t
procedurea under nvrmal clrcunstances. This Is
fine
for
small
proQroms
(under
)000
linea), but when one starts to get serlous
About
an
application, one really needs more segment procedures. On~ reason for this
Is that tHe UCSD Pascal sepArate compilation construct {lNIT:d uses one of
these alx s8cp'nent procedure 'slots' "ven If It Is not loaded Into memory
dynamically, wasting this scarce resource. Thla situation should
lmorove
very soon (perhaps by thB time .thls article Is printed) becRusf! Softech Is
plannlnq to announce features In UCSD Pascal Version IV.O to solve some of
these problems.

~INE("ptr,(-2),LPRfSTO)

When a segment procedure Is called by the UCSD Pascal p-machlne, R soeclal
op-code la used to do so. This op-code flrlt looka In an operatlnq syst~
'segment table' to see If the Indicated procedure has been called more than
zero times (reference count, In case It Is a .recurslve call . and thp. codp.
need not' be reloaded).
If It Is already In memory (reference count) n),
It Is executed like any other procedurel If not, the op-code loo'1

Thl&

article Is copyrighted hy Mlcrosystems (Aox 1192,
NJ, 070Q2) and Is . reprinted with their
permission.

~untalnslde,

One of
the nl~e
features of UCSD Pascal
Is
Its support of seqnent
procedures. A seqment procedure Is I Ike any other Poscal orocedure except
that whenever
It Is called (except for recursive calls) It Is loaded from
disk Into memory prior to being e)(ecuted, and after It exits, the mernory Is
reclaimed.
In fact, the seqnent procedure Is loarled onto the stack,
since
the pattern of memory use Is nested In 8 very 'stack-like' mAnner.
Seqnent
procerlures al low the proqr~er
to manaqe m~nory resources
explicitly and conveniently, and really 8re a form of overloy.
In
IRroe
programs,
It
is not uncomnon to dedicate a seqment procedlJrp. to
Initialization, since that code need not reslrle in memory after the proqr~
starts. Seqment procedures ~y have Internal procerlures anrl functions, All
of which are loaded with the seqnent procedure code, allowlno functional
groups of
routines to be hrouqht Into mfflnory In A slnfJle operation, anrl
executed.

:v
i-*
~

for the segment procedure, and the n~ber of bytes In the code.
It
thp.n
loads
the code onto the stack and cal Is the procedure, Incr~entln~ th~
reference count to one. Upon exit, the seqnent procedure return on-corie
decrements
the reference count by one, and clears the atAck hAck un to
where It was prior to the call If the count has hec~ zero.
The Idea which I had was to somehow write data
Into the sp.qnent
tAhlps
p r lor
t o e a c 1'1 s e!JYl6 n t pro c e d u r e c a I I Ins u c h a wa y t hAt wh e nth eo A h 0 V t'
op-code was
Invoked It would find data descrlblnQ different
st'fJ'TlPnt
procedures each time; It would In fact be faked Into loadlnqdlffprent corle
se9'flents
Into memory for each call, even thouqh the same seO'Tlent nrocf'rillre
was being called each time.
In order to do this, I first had to find where the Beqnent
tRhles wer"
located In memory.
The first fllobal variable declared In the lrc;O PASCfll
operating system code III a pointer to R special recorc1, callec1 the syst~
c~nlC8tlon
record, or SYSonMREC, And the seqnent tahles Are a nRrt of
t h I II r e cor d . The rei s asp e c I R I ' s wit c 1'1' I nth P. I CSO P R S Ca I c omn I I P. r
( t 1'1 f'
'U'
switch), end when Its value Is '-', proqrAms function very c1lffp.rpntly
than usual. The 'main proqram' does not execute at all. hut
rAther
thE!
fir 8 t 9 e qme n t
pro c e d u r e d e c I are d i n the 0 roC) r Aln e l( e c II t e s . I nil t p a c1 • A Iso,
the variable definitions are 'al lased' on too of thp. definitions
for
the
operatlnq system (like an EaIIVALF:N':f: In rffiTRAN), allowln,! the nrol)rAm to
read and write those varlahles.
Usually,
one uses exactly the SWTle
varlahle definitions as were ulled when the operatlnll sy't~n was compllpc1.
In order that one's proQrAm aqree with the operatlnll systrm rlerlnltlons.
In this CAse, however, I simply made my own definitions. since All I wanterl
was to determine the VAlue of the oolnter to SYSonMRfr..
Slnr~ l.S stores
pointers AS Rctual fllP.fYIory addrellscs. hv prlntlnll the VAlue of ttlf' pointer I
COli I d de t e rml ne whe rei n I11P.fTlO r v SYS('"'('l.-Rfr. WA II at 0 r pd.

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1 wrote the following proqr9m and was able to locate SYSOOVREC.
It was
part trial
and error, since after I thought I had 'found' SYSCOMREC the
first time, I was forced to I,ook through dtmps to flqure out why I had been
wrong ••• Anyway, the program below wll I
tell
you where
In memory any
SYSCOMREC 19 under Version 1.5 (It Is at location 718 rdeclmall for my Z-AO
verllon of 1.5), and probably under Version 11.01 I have not tried It with
lit. O.

f su-I

progr9m find;
var
I 1 I n t e 9 e r; f a I I as edt 0 ~ s YBC omr e c

!l t: YJ.ll '" I > I • t.: u u t: u t: II C • C U U tl I u I I l J .
UII C C IJ U I U c:; L II U I I lJ"
that pointer with the fol lowing record definition:

L II '"

LUI I

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\lit"'''\''';

til

I.

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I."
f.,I, ",'

var

',le

r·~.i.'.\.'

alias: record case boolean of
true: (I
Integer);
falser (p : "segtab);
end;
This
record definition states that one will either use the storaqe for the
record 'alias' a8 an Inteqer (denoted 'allss.I') or as a pointer to a
variable of
type
'segtab' (denoted 'allas.p'). Since the same storaqe Is
used for both values, If one were to write Into the Inteqer part, one could
then use that value as a pointer; one could
'fake' Pascal
Into thlnklnq
that a variable of type 'segtype' waB being pointed to.

scqnent procedure flndsyscom;
begin
wrlteln('Syscomrec Is located at address ',1,' decimal.');
end;

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By stating 'aIl8s.1
1= 710 + 9(;', I could access the actual operating
system segment table es 'allas.p"'.
I could then write .Into
the 8eqTIent
table entries of any se~nt which I chose, forcing the syst~n to load the
code I wanted to when I called the appropriate procedure.

beqln
end.
then needed to flqure out how far from the start of SYSOOMREC the Begment
lAhles started. fortunately, the UCSD Pascal
operating ByBten variable
definitions,
found In 8 file called GLOAALS.TEXT, were distributed with
lCSO Pascal Versions 1.4 and 1.5, so t had th~ at my disposal.
fThe

Suppose, for Instance, that I knew that I wanted to exeGute each of
a
sorlos of
ten segment procedures which were located on 'unltr\l', had
length 'Iengthrll' bytes, and otarted at disk block nunber 'block f \l'.
could write a procedure to perform a 'call' to the 'I_th' such segnent

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Copyright 1981 by Mlcrosystens

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Virtual Segment Procedures under UCSO Pascal

Page J

n_OMl.S.TEXT file,
along with all
other UCSD Pascal
source code, 'Is
cooyrlght by the University of California at. San Olegol
some of
the
(".LOMLS.TEXT file
Is presented In this article (the portions of Pascal
!lource In capltel letter9) with the permission of Softech Mlcrosysterns Inc,
their licensee. 1 I om Infor~d that
the SYSOOMREC data layout has not
chanqed with the varlou9 UCSD Pascal Versions, so you should be able to
locate the se~nt tables at 96 bytes past the slart of your SYSCOMREC.
ThIs ~ans
that
If one were to write to location RlA tn my memory, one
would b, wrltlnq on the first byte of the selJl1ent tables,
The

se~nt

tables are defined as followsr

type
seQdesc = record
dlskaddr
inteqerJ
codelenq
Integerl
endl

absolute block number on disk I
In bytes 1

seqtab = array rse9ranqel of record
unit: unltnum; , disk unit number (an Integer) I
codedesc : segdesc; f 8S above l
.
end;
where 'seQrange' Is the numher of seqnent procedures defined for
the UCSO
system belnQ used ('0 .. 15' In the case of Version 1.,).
If one declared a
pointer 'segptr",whlch oolnted to a record of
type
'scqtab", one could
refer
to that
record as 'seqptr", to the unit (disk drive) on which the
code for the third seqrnent procedure was loc8tnd a9 'setlplr"r11.unlt', and
to
the
nU'llber
of
hytes
In
the
code
for
that oror.erfllre fill

Virtual 5eqTIent Procedures under lCSD Pascal

PaQe 4

procedure as folloW81

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program test;
type
eeqdesc = record
dlskaddr
Inteqerl
absolute block ntmber on disk I
codelenq : InteQor;
In bytes 1
end;
seqtab = array fsegranqel of record
unit: unltnun; I disk unIt number
codedeec : segdesc, f 00 above 1
end;
var
al Iss: record csse boolean of
true: (I : InteQer);
falsel (p : Aaegtab);
end;
unit. : arrayfl •. lOl of Inteqer;
lenQ(h : arrayfl .• 101 of Inteqerl
hlock I ar~ayrl •• JOl of lnteqer;
I : Inteqer;
sef1T1ent procecfure 'vl rtual;
beQin f need not have any code, since It wi I I never exccute -the other ten scr,nr.nt procedurcs will execute Instead'
enOl f virtUAl 1
procedure rlovlrtunl(i

If;

intetler);

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begin
r set up to call vi rluat

segment

allas.p~rlO1.codeunit := unitfilj
alias.p~r101.codedesc.codelenq :=
alias.p~r101.codedesc.diskaddr :=

f call virtual seqment
vi rtual;
endj
dovirtual

At

1
Virtual

lengthrilj
blockf'ilj
loaded above'l

CO\!ST
/V1I\XLN IT = 1 2 i
/V1I\XO I R = 77;
YIDLEI'-G = 7;
TIDLEI'G = 15;
FRlKSIZE
512;
OIR8LK
2;
/V1I\XSEG = lS;

r

begin
f ~ e t up poi n t e r
al ias. i := 718 +
f call the ten vi
for i := 1 to 10
end. r test l

tor e a I

s e qrne n t

t a hie

=

9f>j

rtual procedures
do dovirtuat(i);

Segment Procedures under LCSO Pascal

Paqe h

(lI-/V1I\XIIVLM PHYS I CAL LNI T 1/ Fffi LREAIJ*)
(*/V1I\?< N...MAER q: ENTRIES IN A DIRECTffiY*)
(1IN..Mf3ER CF Q-ll\RS I N A VOLUv£ 10*)
(*N...MRER CF OHARS IN TITLE ID*)
(*STANJARD 01 SI< AUXJ.< LEf'.GTH*)
(*01 SK ACCR CF oIRECTffiY*)
(*M4X a::I)E SEOvF.NT N..MAER*)

'~t

TYPE

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The above program will actually work, but it has a few problems which make
it a bit
awkward
to
use.
First off, procedure cal Is which previously
looked I ike a nice nrune
now are
reduced
to
a
cryptic
statement
I ike
'dovirtual(:n<
This
can be
taken care of by cre~tinq constants at the
start of the program with values from 1. to 10, and callinq 'dovirtual' with
those constant values; a pro~edure to clear the screen mlqht then be cal led
as 'dovirtual(clrscreen)', a significant improvement.
The other problem is
that
it
is I\OT EASY
to
find
out
some
of
the
information which
I so casually stated would be found in the 'length' and

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Copyright 1981 by Microsystems

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(*0 IMJL IES DATE !\oT MEANIWFUL'*)
(*oAY CF M)\,JTHIt)
(*100 IS TEMP DISK FLAr,*)

Virtual Seqrnent Procedures under LCSo Pascal

Page 5

'block' arrays.
To do so involves reading the 'segnent tahle' of the
code
f iIe
I n wh I c h
0 ne
0 f the v I r t u a I s e cpne n t sex i s t s , ' I nor de r tad e term i n e
Un for tun ate I y, the dis k add res s i n forma t Ion
s tor e d i n
t hat in forma t ion.
the segment tables of a code file is slightly different than that stored in
the operatinq system's seqrnent tables.
The normal code file disk a~dresses
are relative to the start of the entire code file; the system addresses are
the a b sol ute
dis k
bI 0 ck
mmb e r • T his me an s t hat i nor de r t 0 can v e r t the
data In the code file's seqment table into 'useful'
Information,
we must
also know the absolute address of the start of the code file.
And in order
to determine this, we must read (and understand) the directory of the disk.
Whew!...
.
i t a s t epa t a t i me, the forma t 0 f a l.£So Pas c a I dis k d ire c tor y i s
T a kin q
given below.
It is a Dortion of
the UCSO GLoAAlS.TEXT
file mentioned
earlier (and is copyrlqht by UCSo).

LN ITN...M = 0 •• Mo.XLN IT ;
Vlo
STRIf'.GfVIDLEf'.Gl;

= O.. ~IR;
= STRIf'.GfTIDLEl'-Glj
FILEKII'D = (LNTYPEDFllE,XDSKFILE,OOO£FILE,TEXTFILE.

olru~E

oIRENTRY = RECORD
DFIRSTALK: INTEGER;
DLASTALK: INTECERj
CASE DFKII'D: FILEKIN) OF
SEaREoIR,
LNTYPEDFILE:
(OVID: Vlo;
DEOVAU<: INTEGER;
CN.JvF I LE S: 0 I RRJ\I'-GE ;
OLOADTIME: INTEGER;
DLASTACDT: oATEREC) j
XOSKFllE,COOEFILE,TEXTFILE,INFOFILE,
DATAFILE,GRAFFILE,FOTOFILE:
(oTlo: TID;
DLASTBYTE: 1., FAU
;:

It;,
.~:

!,~ ~

=

var
directory

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INFOFILE,DATAFILE,GRAFFILE,FOTOFILE,SECUREoIR);

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'1:.{.

TID

....•.

'~

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=

oATEREC
PACKED RECORD
M:NTH: 0 ...\2;
DAY: 0 •• 31 j
YEAR: 0 .. 100
EI'D (lIoATEREC*)

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Virtual Seqnent Procedures under UCSO Pascal

Virtual Segnent Procedures under UCSO Pascal
of the last modification to the file. The ahove definitions also tell us
where to find the directory, n~ely at 'dlrblk', or block two.
We can read the directory Into memory with the statenent
~

unltread(unitnun,dlrectory,slzeof{dlrectory),dlrblk);
This statement uses two UCSO Intrlnslcs, the 'slzeof' function and the
'"nltread' procedure.
The former returns the ntrnber of bytes In a Qlven
data structure; In our case, It Is the nunber of bytes In the 'directory'.
The 'unltread' procedure reads from unit (disk drive) 'unltnun' Into ~ry
At
the address of the 'directory' structure for 'slzeof(dlrectory)' bytes
slartlng at absolute disk block '~Irblk'
VI I t

h t h I II I n forma t Ion I n memo r y, we can sea r c h the d \ r e c tor y for an en try
for a given file.
Suppose that we wanted to see If the file 's~ple.code'
were In the directory. We could say
nlrnflles 1= dlrectoryrOl.dnuniTI~B; f nunber of valid entries
I := 0; , will Index Into directory 1
done := false; f will become 'true' when we are done 1
while (I < nunflles) and not done do beqln
I

: = I + 1;

If (dlrectoryfll.dtld = 's9mple.code') then done
end;

:=

truel

If we COllie out of

this loop with 'done' having a value of 'true',' then we
have Indeed found an entry for the required fllel If not, the file was not
present.
If It was found, then the absolute address of the first block of
the file on disk
Is simply 'dlrectoryrll.dflratblk', We have then found
the first thing we needed. the Absolute disk address of' the code file.
We
now nee d tog e t the cod e len g tho n d rei a t I ve dis ked d r e as I n forma t Ion from
the code file's seqnent tables.
The format of the first bloc~ of a code file Is as shown below.
previous type definitions hold a8 before.
segtable

where

the

•

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Page 7
Paqe A

find;
var
codeflle I fllel
.
gegteble : array f segranqel of seqde9cr , 'seqdeac' defined as hefore 1
I ! Integer;
begin
un It: = s;
r read In segnent table -- AS9t.rne 'olrectory' read In as ahove first,
And 'dlrectoryr 11'. Is data for file which Is of Interest to U9 1
unltread(unlt. segtable, slzeof(seqtalJle), dlrectorfll.dflrstblk);
length 1= seqtablefl0 1 .codelenq;
block := seqtableflOl.dlskaddr; , relative block nunher I
block 1= block + dlrectoryfll.dflrstblk; f absolute block ntrober I
end. f find 1

':,'1"

progr~

So, we finally have the entire ball of wax. We can r.ead a l.cSD disk
directory to find out where on disk a file
Is storedl we can read and
understand lhe Information In the seqnent table of a code file; we ca~
convert the relative block nunhers In the code, file seO'l1ent tahles to
absolute block ntrobers; and we can use that InformatIon to Invoke virtual
s e gne n t pro c e d u res • Tot lee v e r y t h I n 9 t 0 q e the r , you w \ \I
fin d he I OW 8
sin g Ie' pro gram wh I c h doe s i t B I I. The pro c e d u r e 'v I r tin I t i n I t I 9 I I z e san
I n t ern a I tab J e 0 fun 1 t, len 9 t h, and b lac kln.f 0 rma t Ion be for e the 0 r 0 Q r am
really gets going.
It does
this by readlnq In the disk directory and
c a II I n g , v I r t II n k' for e 8 c h v I r t u a I s e erne n t pro c e d u r e wh I c h i t w I II c a I I
later.
The 'vlrtllnk' procedure looks In the directory for the code file
requested and puts the required Information In the Internal
table.
The
main program then Invokes the virtual seqnents as 8 teat. Following thIs
program Ie 8 sample of one of the programs which defines a virtual
eeqnent
pro c e d u r e •
No t e t hat I n bot h the p r oq r am wh I c h c a I 1st h e v I r t u a I s e qne n t s
and the program which defines one of th~. the virtual seQments Are defIned
08 the first executable code In the file.
This
Is necessary. alnce the
technique which I have shown requires that both seqnent procedures have the
Bame 'segnent nunbers'.

[
J"'./ ..

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arrayfseqranqel of aegdesc;

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we ~9nted to find the necessary Information about a particular code
Ie, we could use the UCSD Pasca' syst~ Intrinsic orocedure 'unltread' to
road the segment table oata from that file Into the above data structure.
The n • If we wan ted the I n forma tl 0 nab 0 u t the ten t h 8 e gne n t pro c e d u rei n a
code file, we could sl~ly use the tenth el~ent of that array.
For
ex~le.
to determine the 'length' and 'block' data for segnent procedure
n IInb e r ten I n a f I lee a II ed's amp Ie. cod e' 0 nun I t f I v e. one c 0 u I d dot h e
followlnq:
If

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VI rlua) Sec,nent Procenures under U::::SO Pascal

Paqe 9

proqrwn vlJeqleatl

~odeunlt

aNST

codedesc
end;

tv1t\>egln
If (vsegsfvaegnuml.codeunlt = 0) then begin
wr I tel n ( • At t emp t toe l( e cut e un I Ink e d v I r t u a I 8 e g numb e r " v 9 e (J n tro, ' . ' );
exlt(dovlrtual)j r not linked I
end;
load segment register 10 with secrnent data fron I'th sec,nent proceciure
allas.p'rIOl := vaegsfvseqnt.rolj
vIrtual;
end; r dovlrtual
procedure vlrtlnlt;
r Initialize tables for call1nq v seqnent procedures \
const
unum = 5; I unit number where v segnent proca are found
var
I : Integer; r temp var
dIrectory I array rdlrrangel of dlrentry; f holds disk directory
numflles I dlrrange; f number of flies In dIsk dlr ,
procedure vlrtllnk(fname
atrlng; vseqnum
vseoranQe);
var
I I Integeq
clone I b~olean;
fir 8 t b I k I I n t e 9 e r; r b I k numb e r 0 f r Irs t b I k 0 f f a k e s e 0 cod e f I I e I
Isegtable I array rseqranqel of seqdescl f holds sPq th\ fr~ disk I
begin
f find file In directory 1
I := 0; done := false:
wh I I e (I <:: n lro f I I e s) 8 n dna t don e dab e q I n
I ; = I + 1;
If (dlrectory f

ll.dtld:: fnwne) then done:= true:
end;
If not done then heqln
wrlteln('unable to find flle ',fnRffiE'!,'.');
exlt(vlrtllnk};
endj

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Virtual Seqment Procedures under t.x:::SD Pascal

Paqe 11

r

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The wondered where I was,

f read in seqnent table 1
flrstblk != directo ry fi 1 .dfirstblk;
un i tread (unum, I s eqt ab Ie, s I z eo f ( I s eqt ah Ie) , fir s l b 11< ) ;
f enter data' from seqment 10 In found fi Ie into v seq table
vsegsrvsegnuml.codeunit := unum;
vsegsfvsegnum1.codeclesc.codelenq := Iseqtable f lOl.codelenq;
vseqsrvsegnuml~codedesc.diskaddr := Iseqtable r l0 1 .diskaddr + firstblk;
wrlteln('Finished with association of file ',fname,'.');
end; f virtllnk ~
begin
f initialize vseq table so that all units are zero (vseg undefined)
for i := n to J) do vseqsfil.codeunit :=0;
f set up pointer to syscomrec's seq table located by 'find' program
al ias. I := 71.A + 9';; I 9(' bytes in syscom ree before seq tables 1
f read directory into memory 1
unitread(unllm,dlrectory,slzeof(directory),dirblk,O);
nlmfiles := directoryrnl.d n umfiles;
writeln('Dlrectory of unit ',unum,' read in.');
, lin k f i len ame s wit h v s e g numb e r s 1
vir t lin k ( , F AI
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