88008866 AASSSSEEMMBBLLEERR MMAANNUUAAL
UUsers GGuide
8086 1 March 1979
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The 8086 Cross Assembler
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1. General Description
The 8086 Cross Assembler is based on the same system that
produced the 8080 and 6800 Assemblers. This system is written in
BCPL and has been transported to a number of machines. In this
particular Assembler , the syntax of the each line has been
defined without reference to the manufacturers Assembler, which
is not available in Cross form. In fact we do not consider the
manufacturers Assembler to be particularily useful for us , as it
is a heavily typed system requiring all variables to be defined
before use. The Linkage Editor on the 360/195 has been used to
provide linking of previously assembled subroutines, and to
provide library services.
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2. Syntax
The general syntax is the same as that for the other
assemblers, this can be seen in the "CERN M6800 Assembler
Manual". In particular , all the Pseudo operations are the same
as in that document and so are the rules for calculating
expressions and for using macros. The error messages produced are
the same and this manual should be consulted whenever this
document does not define usage of a particular operation. The
manufacturers manual "The MCS-86 USERS MANUAL" should be
consulted to see what the instructions do.
The following symbols still have the same meaning as in the
6800 Assembler.
$ The current value of the PC
# Introducing a Hex constant
" Introducing and ending either
(a) astring
(b) a single character
! An immediate (literal value 8 or 16 bits)
* Introduces a comment line
8086 2 March 1979
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The 8086 Cross Assembler
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3. Instruction Syntax
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3.1 General
The general format of each instruction line is
LABEL OPERATOR OPERAND COMMENT
where the various fields are seperated by spaces and where LABEL
if present must begin in column 1. Multiple spaces are equivalent
to single spaces and spaces are significant in that they
deliniate each field.
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3.2 Operators
These are the same as in the "MCS-86 USERS MANUAL" except
for a few particular instrucions that are mentioned below.
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3.2.1 CALL and JMP Instructions
CALL FRED Direct within segment
JMP FRED
CALLI IFred Indirect within segment
JI IFred
CALLS Fredoffest,Segment Direct Inter-segment
JMPS Fredoffset,Segment
CIS Freddescriptor Indirect Inter-segment
JIS Freddescriptor
B Fred Forces a short jump within segment
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3.2.2 RET Instruction
RET Within segment
RETS Inter-segment (Either of the above instructions
may have an offset added to them)
8086 3 March 1979
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The 8086 Cross Assembler
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3.3 OPERANDS
This structure has one of three main forms
a) No Operand field
b) A single operand
c) Two operands seperated by a comma
The syntax of this part of the instruction is quite
complicated and so i will give an expanded BNF description with
some examples. The symbol ↑ means a null opernad and
<operand>::= <operand main>
<byte prefix>::= &
<operand main>::= <term>|<adr><term>|<adr>,<adr><term>
<adr>::= <register>|<registerbase>|<immediate operand>|
<relitem>|
<absitem>
<register>::= <8bitregister>|
<16bitregister>|
<segmentregister>
<8bitregister>::= AL|AH|BL|BH|CL|CH|DL|DH
<16bitregister>::= AX|BX|CX|DX|SP|BP|SI|DI
<segmentregister>::= CS|DS|ES|SS
<immediateoperand>::= !<absitem>|!<relitem>
<registerbase>::= (BX)|(BP)|(SI)|(DI)|
(BX,SI)|(BX,DI)|(BP,SI)|(BP,DI)
<absitem>::= Absolute expression involving no relocatables
<relitem>::= Relocatable expression (see manual)
<term>::= ↑|<lock><term>|<byte><term>|<seg><term>
<lock>::= [L......]
<byte>::= [B......]
<seg>::= [<segmentreg>]
8086 4 March 1979
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The 8086 Cross Assembler
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3.3.1 Examples
MOV AL,AH Move AH to AL
MOV AX,!#32 Move hex 32 to AX
MOV BX,!FRED Move address of FRED to BX
MOV FRED+2,!2 Move 2 to FRED+2
MOV &FRED+2,!2 Move 2 to byte at FRED+2
MOV FRED+2,!2[B] Same as above
XCHG SEM,AX[LOCK] Lock bus and exchange SEM and AX
JMP FRED Jump to location FRED
RET Return from subroutine
MOV AX,(BP)[DS] Move indirect from BP but force it
into data segment
ADD AX,2(BX) Add AX+(BP+2)=>AX
SUB FRED(SI),AX FRED+(SI)=>FRED(SI)
MOV AL,!"A" Character A into AL
CALLS 10,#400 Segnent Indirect. IP=10and CS=#4000
REP Repeat next instruction
MOV FRED+2-ABST(BP,SI),!FRED+3-#40[B][L][ES]
Yes it is possiblebut I dont know
what it means either
Not all of the syntactic forms are allowed in each
instruction and the requirements for each type of instruction are
given below.In general however for the two address form must
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either be a register or an immediate operand. If it is an
immediate operand it does not matter which way you write it .ie.
MOV !2,AX
MOV AX,!2 are the same.
8086 5 March 1979
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The 8086 Cross Assembler
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3.3.2 Individual Instructions
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3.3.2.1 MOV
All the two address forms are allowed. E.g.
MOV AX,FRED(SI)[L]
MOV MABEL,!2
MOV (BX,SI),BP
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3.3.2.2 PUSH,POP,INC,DEC,NEG,MUL,IMUL,DIV,IDIV,NOT,JI,CALLI,ESC
←←
Only the single address form can be used and no immediate
operands are allowed. PUSH and POP cannot work on byte registers
or addresses, if these are used the word form will be taken.
POP BP
PUSH (BP)
MUL 2(BX)
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3.3.2.3 XCHG,LEA,LDS,LES
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All the two address forms apart from immediate operands are
allowed.
XCHG FRED+2,BX
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3.3.2.4 IN,OUT,INW,OUTW
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Single address form which must be either an absolute value
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les than 255 or there must be no operand.
IN #E3
OUTW
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3.3.2.5 ADD,ADC,SUB,SBB,CMP,TEST,AND,OR,XOR
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All the two address forms except that the register operands
must not be segment registers.
ADD FRED,!FRED
TEST AL,AH
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3.3.2.6 SHR,SHL,SAR,ROL,ROR,RCL,RCR
Either one or two address forms are allowed. The first
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address must not be an immediate operand, and the second address
←←
if present must be CL.
RCR AX
SHR FRED+2,CL
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3.3.2.7 CALL,JMP,JE,JZ....etc
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This is a single address form which must be a relocatable
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item. This mut contain only labels within the program and
absolute numbers.
8086 6 March 1979
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The 8086 Cross Assembler
JNE MABEL
JG MABEL-3
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3.3.2.8 CALLS,JMPS
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This must be a two address form, where both operands must be
absolute items.
CALLS 0,#300
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3.3.2.9 INT
Either one address or none. If it is an address it must be <
255.
INT
INT #12
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3.3.2.10 All other instructions
Thease do not have any operands
8086 7 March 1979
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The 8086 Cross Assembler
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3.4 Byte and Word Addressing
In many cases which involve both byte and word operands, the
assembler can correctly deduce which the operand should be , by
looking at the register type. E.g.
MOV AX,FRED
AND BX,!2
will both require word operands. In the first example a 16 bit
word starting at address FRED will be moved into the 16bit
accumulator,AX. In the other example, the immediate operand 2
will be forced tobe 16 bits long. And similarily
MOV AL,FRED
ADD BX,!2
will work on byte operands.In the first example the 8bit byte
starting at FRED is moved into the lower 8bit accumulator.
For some instructions, however, it is not obvious, wether
the operand should be 16 ior 8 bits long. The following type of
instructions involving an immediate operand are a particular set
of instructions of this type.
MOV FRED,!2
ADD FRED,!#40
Should these be treated as bytes or words? In this assembler such
instructions are, by default, treated as 16 bit words , if you
wish to move 2 into the byte starting at FRED you must write
MOV &FRED,!2
or MOV FRED,!2[BYTE]
or MOV FRED,!2[B]
(the letters after the B in square brackets are irrelevant)
8086 8 March 1979
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The 8086 Cross Assembler
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3.5 The LOCK mechanism
The 8086 has a mechanism for locking a bus while a processor
makes an important transaction that must not be interfered with.
It can do this by prefixing an instruction with a special byte
which prevents any other processor from using the bus until the
following instruction has completed. This is different from the
normal situation where processors compete on a cycle by cycle
access to the bus. The syntax used by this assembler to assert
this locking mechanism is to add a suffix to the operand
expression of the form [L....] (letters after the L are
irrelevant. ) . Thus
XCHG AX,SEMAPHORE[LOCK]
This suffix may occur in any order with the byte suffix, [B...],
or the segment suffix [<segment register>]
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3.6 The segment suffix
In order to force the assembler to take its operand from its
default segment (usually the data segement ,DS) it is nessecary
to add another prefix to the instrucion involved. Again a suffix
in the assembler of the form
[<segment register>]
must be used. Thus
MOV FRED,!2[ES] will move 2 into the address FRED as
an offset in the Extra Segment.Any
of the segments may be used by using
[ES],[DS],[SS],[CS].Thus forcing the
processor to take or put operands in
a different segment.
In particular, instructions using BP as a base register use the
Stack Segment , by default. To force the Efffective address into
the Data Segment we must write
MOV AX,8(BP)[DS]
8086 9 March 1979