UNIX Assembler Reference Manual
Dennis M. Ritchie
Bell Laboratories
Murray Hill, New Jersey

0. Introduction
This document describes the usage and input syntax of the UNIX PDP-11 assembler as. The details of the PDP-11 are
not described; consult the DEC documents ‘‘PDP-11/20 Handbook’’ and ‘‘PDP-11/45 Handbook.’’
The input syntax of the UNIX assembler is generally similar to that of the DEC assembler PAL-11R, although its internal workings and output format are unrelated. It may be useful to read the publication DEC-11-ASDB-D, which describes PAL-11R, although naturally one must use care in assuming that its rules apply to as.
As is a rather ordinary two-pass assembler without macro capabilities. It produces an output file which contains relocation information and a complete symbol table; thus the output is acceptable to the UNIX link-editor ld, which
may be used to combine the outputs of several assembler runs and to obtain object programs from libraries. The
output format has been designed so that if a program contains no unresolved references to external symbols, it is executable without further processing.
1. Usage
as is used as follows:
as [ − ] file1 . . .
If the optional ‘‘−’’ argument is given, all undefined symbols in the current assembly will be made undefined-external. See the .globl directive below.
The other arguments name files which are concatenated and assembled. Thus programs may be written in several
pieces and assembled together.
The output of the assembler is placed on the file a.out in the current directory. If there were no unresolved externalreferences, and no errors detected, a.out is made executable; otherwise, if it is produced at all, it is made non-executable.
2. Lexical conventions
Assembler tokens include identifiers (alternatively, ‘‘symbols’’ or ‘‘names’’), temporary symbols, constants, and operators.
2.1 Identifiers
An identifier consists of a sequence of alphanumeric characters (including period ‘‘ . ’’, underscore ‘‘ ’’, and tilde‘‘˜’’ as alphanumeric) of which the first may not be numeric. Only the first eight characters are significant. When aname begins with a tilde, the tilde is discarded and that occurrence of the identifier generates a unique entry in thesymbol table which can match no other occurrence of the identifier. This feature is used by the C compiler to place
names of local variables in the output symbol table without having to worry about making them unique.
2.2 Temporary symbols

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A temporary symbol consists of a digit followed by ‘‘f ’’ or ‘‘b’’. Temporary symbols are discussed fully in §5.1.
2.3 Constants
An octal constant consists of a sequence of digits; ‘‘8’’ and ‘‘9’’ are taken to have octal value 10 and 11. The constant is truncated to 16 bits and interpreted in two’s complement notation.
A decimal constant consists of a sequence of digits terminated by a decimal point ‘‘.’’. The magnitude of the constant should be representable in 15 bits; i.e., be less than 32,768.
A single-character constant consists of a single quote ‘‘ ′ ’’ followed by an ASCII character not a new-line. Certain
dual-character escape sequences are acceptable in place of the ASCII character to represent new-line and other nongraphics (see String statements, §5.5). The constant’s value has the code for the given character in the least significant byte of the word and is null-padded on the left.
A double-character constant consists of a double quote ‘‘ " ’’ followed by a pair of ASCII characters not including
new-line. Certain dual-character escape sequences are acceptable in place of either of the ASCII characters to represent new-line and other non-graphics (see String statements, §5.5). The constant’s value has the code for the firstgiven character in the least significant byte and that for the second character in the most significant byte.
2.4 Operators
There are several single- and double-character operators; see §6.
2.5 Blanks
Blank and tab characters may be interspersed freely between tokens, but may not be used within tokens (except character constants). A blank or tab is required to separate adjacent identifiers or constants not otherwise separated.
2.6 Comments
The character ‘‘ / ’’ introduces a comment, which extends through the end of the line on which it appears. Comments are ignored by the assembler.
3. Segments
Assembled code and data fall into three segments: the text segment, the data segment, and the bss segment. The text
segment is the one in which the assembler begins, and it is the one into which instructions are typically placed. The
UNIX system will, if desired, enforce the purity of the text segment of programs by trapping write operations into it.
Object programs produced by the assembler must be processed by the link-editor ld (using its ‘‘−’’ flag) if the text
segment is to be write-protected. A single copy of the text segment is shared among all processes executing such a
program.
The data segment is available for placing data or instructions which will be modified during execution. Anythingwhich may go in the text segment may be put into the data segment. In programs with write-protected, sharable text
segments, data segment contains the initialized but variable parts of a program. If the text segment is not pure, thedata segment begins immediately after the text segment; if the text segment is pure, the data segment begins at thelowest 8K byte boundary after the text segment.
The bss segment may not contain any explicitly initialized code or data. The length of the bss segment (like that oftext or data) is determined by the high-water mark of the location counter within it. The bss segment is actually an
extension of the data segment and begins immediately after it. At the start of execution of a program, the bss segment is set to 0. Typically the bss segment is set up by statements exemplified bylab: . = .+10
The advantage in using the bss segment for storage that starts off empty is that the initialization information need not
be stored in the output file. See also Location counter and Assignment statements below.
4. The location counter
One special symbol, ‘‘ . ’’, is the location counter. Its value at any time is the offset within the appropriate segmentof the start of the statement in which it appears. The location counter may be assigned to, with the restriction thatthe current segment may not change; furthermore, the value of ‘‘ . ’’ may not decrease. If the effect of the assign--Assembler Manual - 3

ment is to increase the value of ‘‘ . ’’, the required number of null bytes are generated (but see Segments above).
5. Statements
A source program is composed of a sequence of statements. Statements are separated either by new-lines or by
semicolons. There are five kinds of statements: null statements, expression statements, assignment statements, stringstatements, and keyword statements.
Any kind of statement may be preceded by one or more labels.
5.1 Labels
There are two kinds of label: name labels and numeric labels. A name label consists of a name followed by a colon( : ). The effect of a name label is to assign the current value and type of the location counter ‘‘ . ’’ to the name. An
error is indicated in pass 1 if the name is already defined; an error is indicated in pass 2 if the ‘‘ . ’’ value assigned
changes the definition of the label.
A numeric label consists of a digit 0 to 9 followed by a colon ( : ). Such a label serves to define temporary symbolsof the form ‘‘n b’’ and ‘‘n f ’’, where n is the digit of the label. As in the case of name labels, a numeric label assigns
the current value and type of ‘‘ . ’’ to the temporary symbol. However, several numeric labels with the same digit
may be used within the same assembly. References of the form ‘‘n f ’’ refer to the first numeric label ‘‘n :’’ f orwardfrom the reference; ‘‘nb’’ symbols refer to the first ‘‘n :’’ label backward from the reference. This sort of temporary
label was introduced by Knuth [The Art of Computer Programming, Vol I: Fundamental Algorithms ]. Such labels
tend to conserve both the symbol table space of the assembler and the inventive powers of the programmer.
5.2 Null statements
A null statement is an empty statement (which may, however, have labels). A null statement is ignored by the assembler. Common examples of null statements are empty lines or lines containing only a label.
5.3 Expression statements
An expression statement consists of an arithmetic expression not beginning with a keyword. The assembler computes its (16-bit) value and places it in the output stream, together with the appropriate relocation bits.
5.4 Assignment statements
An assignment statement consists of an identifier, an equals sign ( = ), and an expression. The value and type of theexpression are assigned to the identifier. It is not required that the type or value be the same in pass 2 as in pass 1,
nor is it an error to redefine any symbol by assignment.
Any external attribute of the expression is lost across an assignment. This means that it is not possible to declare a
global symbol by assigning to it, and that it is impossible to define a symbol to be offset from a non-locally defined
global symbol.
As mentioned, it is permissible to assign to the location counter ‘‘ . ’’. It is required, however, that the type of the expression assigned be of the same type as ‘‘ . ’’, and it is forbidden to decrease the value of ‘‘ . ’’. In practice, the mostcommon assignment to ‘‘ . ’’ has the form ‘‘. = . + n’’ for some number n; this has the effect of generating n nullbytes.
5.5 String statements
A string statement generates a sequence of bytes containing ASCII characters. A string statement consists of a left
string quote ‘‘<’’ followed by a sequence of ASCII characters not including newline, followed by a right string quote‘‘>’’. Any of the ASCII characters may be replaced by a two-character escape sequence to represent certain nongraphic characters, as follows:-

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\n
\t
\e
\0
\r
\a
\p
\\
\>

NL
HT
EOT
NUL
CR
ACK
PFX

(012)
(011)
(004)
(000)
(015)
(006)
(033)

\
>

The last two are included so that the escape character and the right string quote may be represented. The same escape sequences may also be used within single- and double-character constants (see §2.3 above).
5.6 Keyword statements
Keyword statements are numerically the most common type, since most machine instructions are of this sort. A
keyword statement begins with one of the many predefined keywords of the assembler; the syntax of the remainder
depends on the keyword. All the keywords are listed below with the syntax they require.
6. Expressions
An expression is a sequence of symbols representing a value. Its constituents are identifiers, constants, temporary
symbols, operators, and brackets. Each expression has a type.
All operators in expressions are fundamentally binary in nature; if an operand is missing on the left, a 0 of absolute
type is assumed. Arithmetic is two’s complement and has 16 bits of precision. All operators have equal precedence,and expressions are evaluated strictly left to right except for the effect of brackets.
6.1 Expression operators
The operators are:(blank)when there is no operator between operands, the effect is exactly the same as if a ‘‘+’’ had appeared.

+

addition−subtraction*

multiplication\/

division (note that plain ‘‘ / ’’ starts a comment)&

bitwise andbitwise or>>

logical right shift<<

logical left shift%modulo!

a ! b is a or ( not b ); i.e., the or of the first operand and the one’s complement of the second;most common use is as a unary.

ˆ

result has the value of first operand and the type of the second; most often used to define newmachine instructions with syntax identical to existing instructions.

Expressions may be grouped by use of square brackets ‘‘ [ ] ’’. (Round parentheses are reserved for address modes.)6.2 Types
The assembler deals with a number of types of expressions. Most types are attached to keywords and used to selectthe routine which treats that keyword. The types likely to be met explicitly are:

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undefined
Upon first encounter, each symbol is undefined. It may become undefined if it is assigned an undefined expression. It is an error to attempt to assemble an undefined expression in pass 2; in pass 1, it
is not (except that certain keywords require operands which are not undefined).
undefined external
A symbol which is declared .globl but not defined in the current assembly is an undefined external. Ifsuch a symbol is declared, the link editor ld must be used to load the assembler’s output with anotherroutine that defines the undefined reference.
absolute
An absolute symbol is one defined ultimately from a constant. Its value is unaffected by any possible
future applications of the link-editor to the output file.
text
The value of a text symbol is measured with respect to the beginning of the text segment of the program. If the assembler output is link-edited, its text symbols may change in value since the program
need not be the first in the link editor’s output. Most text symbols are defined by appearing as labels.
At the start of an assembly, the value of ‘‘ . ’’ is text 0.
data
The value of a data symbol is measured with respect to the origin of the data segment of a program.
Like text symbols, the value of a data symbol may change during a subsequent link-editor run sincepreviously loaded programs may have data segments. After the first .data statement, the value of‘‘ . ’’ is data 0.
bss
The value of a bss symbol is measured from the beginning of the bss segment of a program. Like text
and data symbols, the value of a bss symbol may change during a subsequent link-editor run, sincepreviously loaded programs may have bss segments. After the first .bss statement, the value of ‘‘ . ’’is bss 0.
external absolute, text, data, or bss
symbols declared .globl but defined within an assembly as absolute, text, data, or bss symbols may be
used exactly as if they were not declared .globl; however, their value and type are available to the link
editor so that the program may be loaded with others that reference these symbols.
register
The symbols
r0 . . . r5
fr0 . . . fr5
sppc
are predefined as register symbols. Either they or symbols defined from them must be used to refer tothe six general-purpose, six floating-point, and the 2 special-purpose machine registers. The behaviorof the floating register names is identical to that of the corresponding general register names; the former are provided as a mnemonic aid.
other types
Each keyword known to the assembler has a type which is used to select the routine which processes
the associated keyword statement. The behavior of such symbols when not used as keywords is thesame as if they were absolute.
6.3 Type propagation in expressions
When operands are combined by expression operators, the result has a type which depends on the types of theoperands and on the operator. The rules involved are complex to state but were intended to be sensible and predictable. For purposes of expression evaluation the important types are
undefined
absolutetext

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data
bss
undefined external
other
The combination rules are then: If one of the operands is undefined, the result is undefined. If both operands are absolute, the result is absolute. If an absolute is combined with one of the ‘‘other types’’ mentioned above, or with aregister expression, the result has the register or other type. As a consequence, one can refer to r3 as ‘‘r0+3’’. If two
operands of ‘‘other type’’ are combined, the result has the numerically larger type (not that this fact is very useful,since the values are not made public). An ‘‘other type’’ combined with an explicitly discussed type other than absolute acts like an absolute.
Further rules applying to particular operators are:+If one operand is text-, data-, or bss-segment relocatable, or is an undefined external, the result has
the postulated type and the other operand must be absolute.

−

If the first operand is a relocatable text-, data-, or bss-segment symbol, the second operand may be
absolute (in which case the result has the type of the first operand); or the second operand may
have the same type as the first (in which case the result is absolute). If the first operand is external
undefined, the second must be absolute. All other combinations are illegal.

ˆThis operator follows no other rule than that the result has the value of the first operand and thetype of the second.

others

It is illegal to apply these operators to any but absolute symbols.

7. Pseudo-operations
The keywords listed below introduce statements which generate data in unusual forms or influence the later operations of the assembler. The metanotation[ stuff ] . . .
means that 0 or more instances of the given stuff may appear. Also, boldface tokens are literals, italic words are substitutable.
7.1 .byte expression [ , expression ] . . .
The expressions in the comma-separated list are truncated to 8 bits and assembled in successive bytes. The expressions must be absolute. This statement and the string statement above are the only ones which assemble data one
byte at at time.
7.2 .even
If the location counter ‘‘ . ’’ is odd, it is advanced by one so the next statement will be assembled at a word boundary.
7.3 .if expression
The expression must be absolute and defined in pass 1. If its value is nonzero, the .if is ignored; if zero, the statements between the .if and the matching .endif (below) are ignored. .if may be nested. The effect of .if cannot extend beyond the end of the input file in which it appears. (The statements are not totally ignored, in the following
sense: .ifs and .endifs are scanned for, and moreover all names are entered in the symbol table. Thus names occurring only inside an .if will show up as undefined if the symbol table is listed.)
7.4 .endif
This statement marks the end of a conditionally-assembled section of code. See .if above.
7.5 .globl name [ , name ] . . .
This statement makes the names external. If they are otherwise defined (by assignment or appearance as a label)
they act within the assembly exactly as if the .globl statement were not given; however, the link editor ld may be
used to combine this routine with other routines that refer these symbols.

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Conversely, if the given symbols are not defined within the current assembly, the link editor can combine the outputof this assembly with that of others which define the symbols.
As discussed in §1, it is possible to force the assembler to make all otherwise undefined symbols external.
7.6 .text
7.7 .data
7.8 .bss
These three pseudo-operations cause the assembler to begin assembling into the text, data, or bss segment respectively. Assembly starts in the text segment. It is forbidden to assemble any code or data into the bss segment, but
symbols may be defined and ‘‘ . ’’ moved about by assignment.
7.9 .comm name , expression
Provided the name is not defined elsewhere, this statement is equivalent to.globl namename = expression ˆ nameThat is, the type of name is ‘‘undefined external’’, and its value is expression. In fact the name behaves in the current assembly just like an undefined external. However, the link-editor ld has been special-cased so that all external
symbols which are not otherwise defined, and which have a non-zero value, are defined to lie in the bss segment, andenough space is left after the symbol to hold expression bytes. All symbols which become defined in this way arelocated before all the explicitly defined bss-segment locations.
8. Machine instructions
Because of the rather complicated instruction and addressing structure of the PDP-11, the syntax of machine instruction statements is varied. Although the following sections give the syntax in detail, the 11/20 and 11/45 handbooks
should be consulted on the semantics.
8.1 Sources and Destinations
The syntax of general source and destination addresses is the same. Each must have one of the following forms,
where reg is a register symbol, and expr is any sort of expression:
syntaxreg( reg ) +− ( reg )expr ( reg )( reg )* reg
* ( reg ) +* − ( reg )* ( reg )* expr ( reg )expr$ expr* expr* $ expr

words

mode

0
0
0
1
0
0
0
0
1
1
1
1
1
1

0+reg
2+reg
4+reg
6+reg
1+reg
1+reg
3+reg
5+reg
7+reg
7+reg
67
27
77
37

The words column gives the number of address words generated; the mode column gives the octal address-mode
number. The syntax of the address forms is identical to that in DEC assemblers, except that ‘‘*’’ has been substitutedfor ‘‘@’’ and ‘‘$’’ for ‘‘#’’; the UNIX typing conventions make ‘‘@’’ and ‘‘#’’ rather inconvenient.
Notice that mode ‘‘*reg’’ is identical to ‘‘(reg)’’; that ‘‘*(reg)’’ generates an index word (namely, 0); and that addresses consisting of an unadorned expression are assembled as pc-relative references independent of the type of theexpression. To force a non-relative reference, the form ‘‘*$expr’’ can be used, but notice that further indirection is-Assembler Manual - 8

impossible.
8.3 Simple machine instructions
The following instructions are defined as absolute symbols:
clc
clv
clz
cln
sec
sev
sez
sen
They therefore require no special syntax. The PDP-11 hardware allows more than one of the ‘‘clear’’ class, or alternatively more than one of the ‘‘set’’ class to be or-ed together; this may be expressed as follows:clc  clv
8.4 Branch
The following instructions take an expression as operand. The expression must lie in the same segment as the reference, cannot be undefined-external, and its value cannot differ from the current location of ‘‘ . ’’ by more than 254
bytes:
br
bne
beq
bge
blt
bgt
ble
bpl
bmi
bhi

blos
bvc
bvs
bhis
bec
bcc
blo
bcs
bes

(= bcc)(= bcs)bes (‘‘branch on error set’’) and bec (‘‘branch on error clear’’) are intended to test the error bit returned by system
calls (which is the c-bit).
8.5 Extended branch instructions
The following symbols are followed by an expression representing an address in the same segment as ‘‘ . ’’. If thetarget address is close enough, a branch-type instruction is generated; if the address is too far away, a jmp will be
used.
jbr
jne
jeq
jge
jlt
jgt
jle
jpl
jmi
jhi

jlos
jvc
jvs
jhis
jec
jcc
jlo
jcs
jes

jbr turns into a plain jmp if its target is too remote; the others (whose names are contructed by replacing the ‘‘b’’ inthe branch instruction’s name by ‘‘j’’ ) turn into the converse branch over a jmp to the target address.
8.6 Single operand instructions
The following symbols are names of single-operand machine instructions. The form of address expected is discussed in §8.1 above.

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clr
clrb
com
comb
inc
incb
dec
decb
neg
negb
adc
adcb
sbc

sbcb
ror
rorb
rol
rolb
asr
asrb
asl
aslb
jmp
swab
tst
tstb

8.7 Double operand instructions
The following instructions take a general source and destination (§8.1), separated by a comma, as operands.
mov
movb
cmp
cmpb
bit
bitb
bic
bicb
bis
bisb
add
sub
8.8 Miscellaneous instructions
The following instructions have more specialized syntax. Here reg is a register name, src and dst a general source ordestination (§8.1), and expr is an expression:
jsr
rts
sys
ash
ashc
mul
div
xor
sxt
mark
sob

reg,dstreg
expr
src , regsrc , regsrc , regsrc , regreg , dst
dst
expr
reg , expr

(or, als)(or, alsc)(or, mpy)(or, dvd)

sys is another name for the trap instruction. It is used to code system calls. Its operand is required to be expressiblein 6 bits. The alternative forms for ash, ashc, mul, and div are provided to avoid conflict with EAE register names
should they be needed.
The expression in mark must be expressible in six bits, and the expression in sob must be in the same segment as‘‘ . ’’, must not be external-undefined, must be less than ‘‘ . ’’, and must be within 510 bytes of ‘‘ . ’’.
8.9 Floating-point unit instructions
The following floating-point operations are defined, with syntax as indicated:
cfcc
setf
setd

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seti
setl
clrf
negf
absf
tstf
movf
movf
movif
movfi
movof
movfo
movie
movei
addf
subf
mulf
divf
cmpf
modf
ldfps
stfps
stst

fdst
fdst
fdst
fsrc
fsrc, freg
freg, fdst
src, freg
freg, dst
fsrc, freg
freg, fdst
src, freg
freg, dst
fsrc, freg
fsrc, freg
fsrc, freg
fsrc, freg
fsrc, freg
fsrc, freg
src
dst
dst

(= ldf )(= stf )(= ldcif )(= stcfi )(= ldcdf )(= stcfd )(= ldexp)(= stexp)

fsrc, fdst, and freg mean floating-point source, destination, and register respectively. Their syntax is identical to thatfor their non-floating counterparts, but note that only floating registers 0—3 can be a freg.
The names of several of the operations have been changed to bring out an analogy with certain fixed-point instructions. The only strange case is movf, which turns into either stf or ldf depending respectively on whether its first
operand is or is not a register. Warning: ldf sets the floating condition codes, stf does not.
9. Other symbols
9.1 . .
The symbol ‘‘ . . ’’ is the relocation counter. Just before each assembled word is placed in the output stream, the current value of this symbol is added to the word if the word refers to a text, data or bss segment location. If the output
word is a pc-relative address word which refers to an absolute location, the value of ‘‘ . . ’’ is subtracted.
Thus the value of ‘‘ . . ’’ can be taken to mean the starting core location of the program. In UNIX systems with relocation hardware, the initial value of ‘‘ . . ’’ is 0.
The value of ‘‘ . . ’’ may be changed by assignment. Such a course of action is sometimes necessary, but the consequences should be carefully thought out. It is particularly ticklish to change ‘‘ . . ’’ midway in an assembly or to do
so in a program which will be treated by the loader, which has its own notions of ‘‘ . . ’’.
9.2 System calls
The following absolute symbols may be used to code calls to the UNIX system (see the sys instruction above).
break
chdir
chmod
chown
close
creat
exec
exit
fork
fstat
getuid
gtty

nice
open
read
seek
setuidsignal
stat
stime
stty
tell
time
umount-

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makdirmdate
mount

unlinkwait
write

Warning: the wait system call is not the same as the wait instruction, which is not defined in the assembler.
10. Diagnostics
When an input file cannot be read, its name followed by a question mark is typed and assembly ceases. When syntactic or semantic errors occur, a single-character diagnostic is typed out together with the line number and the filename in which it occurred. Errors in pass 1 cause cancellation of pass 2. The possible errors are:
)
]
>
*
.
A
B
E
F
G
I
M
O
P
R
U
X

parentheses error
parentheses error
string not terminated properly
indirection ( * ) used illegallyillegal assignment to ‘‘ . ’’error in address
branch address is odd or too remoteerror in expression
error in local (‘‘f ’’ or ‘‘b’’) type symbol
garbage (unknown) characterend of file inside an .if
multiply defined symbol as label
word quantity assembled at odd addressphase error—‘‘ . ’’ different in pass 1 and 2
relocation error
undefined symbol
syntax error