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OUTLINE
 2.1 Basic Assembler Functions
◦ A simple SIC assembler
◦ Assembler tables and logic
 2.2 Machine-Dependent Assembler Features
◦ Instruction formats and addressing modes
◦ Program relocation
 2.3 Machine-Independent Assembler Features
 2.4 Assembler Design Options
◦ Two-pass
◦ One-pass
◦ Multi-pass

2.1 BASIC ASSEMBLER FUNCTIONS
 Figure 2.1 shows an assembler language program
for SIC.
◦ The line numbers are for reference only.
◦ Indexing addressing is indicated by adding the
modifier “,X”
◦ Lines beginning with “.”contain comments
only.
◦ Reads records from input device (code F1)
◦ Copies them to output device (code 05)
◦ At the end of the file, writes EOF on the output
device, then RSUB to the operating system

 Assembler directives (pseudo-
instructions)
◦ START, END, BYTE, WORD, RESB,
RESW.
◦ These statements are not translated into
machine instructions.
◦ Instead, they provide instructions to the
assembler itself.

 Data transfer (RD, WD)
◦ A buffer is used to store record
◦ Buffering is necessary for different I/O rates
◦ The end of each record is marked with a null
character (0016)
◦ Buffer length is 4096 Bytes
◦ The end of the file is indicated by a zero-length
record
◦ When the end of file is detected, the program
writes EOF on the output device and terminates
by RSUB.
 Subroutines (JSUB, RSUB)
◦ RDREC, WRREC
◦ Save link (L) register first before nested jump

2.1.1 A SIMPLE SIC ASSEMBLER
 Figure 2.2 shows the generated object code for
each statement.
◦ Loc gives the machine address in Hex.
◦ Assume the program starting at address 1000.
 Translation functions
◦ Translate STL to 14.
◦ Translate RETADR to 1033.
◦ Build the machine instructions in the proper
format (,X).
◦ Translate EOF to 454F46.
◦ Write the object program and assembly listing

Forward reference problem
 Reference to a label that is defined later
in the program.

 A forward reference
◦ In line number10
1000 FIRST STL RETADR 141033
◦ A reference to a label (RETADR) that is
defined later in the program
◦ Most assemblers make two passes over the
source program
 Most assemblers make two passes over source
program.
◦ Pass 1 scans the source for label definitions
and assigns address (Loc).
◦ Pass 2 performs most of the actual translation.

 Assembler’s Functions
◦ Convert mnemonic operation codes to their
machine language equivalents
 STL to 14
◦ Convert symbolic operands (referred label)
to their equivalent machine addresses
 RETADR to 1033
◦ Build the machine instructions in the proper
format
◦ Convert the data constants to internal
machine representations
◦ Write the object program and the assembly
listing

The functions of the two passes assembler.
 Pass 1 (define symbol)
◦ Assign addresses to all statements (generate LOC).
◦ Check the correctness of Instruction (check with OP table).
◦ Save the values (address) assigned to all labels into
SYMBOL table for Pass 2.
◦ Perform some processing of assembler directives.
 Pass 2
◦ Assemble instructions (op code from OP table, address
from SYMBOL table).
◦ Generate data values defined by BYTE, WORD.
◦ Perform processing of assembler directives not done
during Pass 1.
◦ Write the OP (Fig. 2.3) and the assembly listing (Fig. 2.2).

2.1.2 DATA STRUCTURES in ASSEMBLER
 Our simple assembler uses two internal tables: The
OPTAB and SYMTAB.
◦ OPTAB is used to look up mnemonic operation codes
and translate them to their machine language
equivalents.
 LDA→00, STL→14, …
◦ SYMTAB is used to store values (addresses) assigned
to labels.
 COPY→1000, FIRST→1000 …
 Location Counter LOCCTR
◦ LOCCTR is a variable for assignment addresses.
◦ LOCCTR is initialized to address specified in START.
◦ When reach a label, the current value of LOCCTR
gives the address to be associated with that label.

2.1.2 ASSEMBLER TABLES AND
LOGIC
 The Operation Code Table (OPTAB)
◦ Contain the mnemonic operation & its
machine language equivalents (at least).
◦ Contain instruction format& length.
◦ Pass 1, OPTAB is used to look up and
validate operation codes.
◦ Pass 2, OPTAB is used to translate the
operation codes to machine language.
◦ In SIC/XE, assembler search OPTAB in
Pass 1 to find the instruction length for
incrementing LOCCTR.
◦ Organize as a hash table (static table).

2.1.2 ASSEMBLER TABLES AND LOGIC
 The Symbol Table (SYMTAB)
◦ Include the name and value (address) for
each label.
◦ Include flags to indicate error conditions
◦ Contain type, length.
◦ Pass 1, labels are entered into SYMTAB,
along with assigned addresses (from
LOCCTR).
◦ Pass 2, symbols used as operands are
look up in SYMTAB to obtain the
addresses.
◦ Organize as a hash table (static table).
◦ The entries are rarely deleted from table.

2.1.2 ASSEMBLER TABLES AND LOGIC
 Pass 1 usually writes an intermediate file.
◦ Contain source statement together with
its assigned address, error indicators.
◦ This file is used as input to Pass 2.
 Figure 2.4 shows the two passes of
assembler.
◦ Format with fields LABEL, OPCODE,
and OPERAND.
◦ Denote numeric value with the prefix#.
#[OPERAND]

2.2 MACHINE-DEPENDENT ASSEMBLER
FEATURES
 Indirect addressing
◦ Adding the prefix @ to operand (line 70).
 Immediate operands
◦ Adding the prefix # to operand (lines 12, 25, 55,
133).
 Extended format
◦ Adding the prefix + to OP code (lines 15, 35, 65).
 The use of register-register instructions.
◦ Faster and don’t require another memory reference.

Figure 2.5: WRREC
Figure 2.5: Example of SIC/XE Program

2.2 MACHINE-DEPENDENT ASSEMBLER
FEATURES
 SIC/XE
◦ PC-relative/Base-relative addressing op m
◦ Indirect addressing op @m
◦ Immediate addressing op #c
◦ Extended format +op m
◦ Index addressing op m, X
◦ register-to-register instructions COMPR
◦ larger memory →multi-programming (program
allocation)

2.2 MACHINE-DEPENDENT
ASSEMBLER FEATURES
 Register translation
◦ register name (A, X, L, B, S, T, F, PC, SW) and
their values (0, 1, 2, 3, 4, 5, 6, 8, 9)
◦ preloaded in SYMTAB
 Address translation
◦ Most register-memory instructions use program
counter relative or base relative addressing
◦ Format 3: 12-bit disp (address) field
 PC-relative: -2048~2047
 Base-relative: 0~4095
◦ Format 4: 20-bit address field (absolute
addressing)

2.2.1 INSTRUCTION FORMATS &
ADDRESSING MODES
 The START statement
◦ Specifies a beginning address of 0.
 Register-register instructions
◦ CLEAR & TIXR, COMPR
 Register-memory instructions are using
◦ Program-counter (PC) relative addressing
◦ The program counter is advanced after each
instruction is fetched and before it is executed.
◦ PC will contain the address of the next
instruction.
10 0000 FIRST STL RETADR
17202D
TA -(PC) = disp= 30H –3H= 2D

Figure 2.6 Program from fig 2.5 with object code

2.2.1 INSTRUCTION FORMATS &
ADDRESSING MODES
40 0017 J CLOOP 3F2FEC
0006 - 001A= disp= -14
 Base (B), LDB #LENGTH, BASE LENGTH
160 104E STCH BUFFER, X 57C003
TA-(B) = 0036 -(B) = disp= 0036-0033 = 0003
 Extended instruction
15 0006 CLOOP +JSUB RDREC
4B101036
 Immediate instruction
55 0020 LDA #3 010003
133 103C +LDT #4096 75101000
 PC relative + indirect addressing (line 70)

2.2.2 Program Relocation
 Absolute program, relocatable program

2.2.2 PROGRAM RELOCATION
 Note that no matter where the program is loaded,
RDREC is always is 1036 bytes past the starting
address of the program. This means that we can solve
the relocation problem in the following way,
1. When the assembler generates the object code for the
JSUB instruction we are considering it will insert the
address of RDREC relative to the start of the
program.
This is the reason we initialized the location counter
to 0 for the assembly
2. The assembler will also produce the command for
the loader, instructing it to add the beginning address
of the program to address the field in the JSUB
instruction at the load time.

2.2.2 PROGRAM RELOCATION
 Modification record (direct addressing)
◦ 1 M
◦ 2-7 Starting location of the address field to be modified,
relative to the beginning of the program.
◦ 8-9 Length of the address field to be modified, in half
bytes.
M^000007^05

2.3 MACHINE-INDEPENDENT
ASSEMBLER FEATURES
 Write the value of a constant operand as a
part of the instruction that uses it (Fig. 2.9).
 A literal is identified with the prefix =
45 001AENDFIL LDA=C’EOF’
032010
◦ Specifies a 3-byte operand whose value is
the character string EOF.
215 1062 WLOOP TD =X’05’
E32011
◦ Specifies a 1-byte literal with the
hexadecimal value 05

WRRE
C
Figure 2.9 Program demonstrating additional assembler features

2.3.1 LITERALS
 The difference between literal operands and
immediate operands
◦ =, #
◦ Immediate addressing, the operand value is
assembled as part of the machine instruction, no
memory reference.
◦ With a literal, the assembler generates the specified
value as a constant at some other memory location.
The address of this generated constant is used as the
TA for the machine instruction, using PC-relative or
base-relative addressing with memory reference.
 Literal pools
◦ At the end of the program (Fig. 2.10).
◦ Assembler directive LTORG, it creates a literal pool
that contains all of the literal operands used since
the previous LTORG.

WRRE
C
Figure 2.10 Program from figure 2.9 with object code

2.3.1 Literals
 When to use LTORG
◦ The literal operand would be placed too far away
from the instruction referencing.
◦ Cannot use PC-relative addressing or Base-
relative addressing to generate Object Program.
 Most assemblers recognize duplicate
literals.
◦ By comparison of the character strings defining
them.
◦ =C’EOF’ and =X’454F46’

2.3.1 LITERALS
 Allow literals that refer to the current value of
the location counter.
◦ Such literals are sometimes useful for
loading base registers.
LDB =*
; register B=beginning address of
statement=current LOCBASE*
; for base relative addressing
 If a literal =*appeared on line 13 or 55
◦ Specify an operand with value 0003 (Loc) or
0020 (Loc).

2.3.1 LITERALS
 Literal table (LITTAB)
◦ Contains the literal name (=C’EOF’), the
operand value (454F46) and length (3),
and the address (002D).
◦ Organized as a hash table.
◦ Pass 1, the assembler creates or searches
LITTAB for the specified literal name.
◦ Pass 1 encounters a LTORG statement or
the end of the program, the assembler
makes a scan of the literal table.
◦ Pass 2, the operand address for use in
generating OC is obtained by searching
LITTAB.

2.3.2 SYMBOL-DEFINING
STATEMENTS
 Allow the programmer to define symbols and
specify their values.
◦ Assembler directive EQU.
◦ Improved readability in place of numeric values.
+LDT #4096
MAXLEN EQU BUFEND-BUFFER
(4096)
+LDT #MAXLEN
 Use EQU in defining mnemonic names for
registers.
◦ Registers A, X, L can be used by numbers 0, 1,
2.

2.3.2 SYMBOL-DEFINING STATEMENTS
 The standard names reflect the usage of the
registers.
BASE EQUR1
COUNT EQUR2
INDEX EQUR3
 Assembler directive ORG
◦ Use to indirectly assign values to symbols.
ORG value
◦ The assembler resets its LOCCTR to the
specified value.
◦ ORG can be useful in label definition.

 The location counter is used to control
assignment of storage in the object program
◦ In most cases, altering its value would
result in an incorrect assembly.
 ORG is used
http://home.educities.edu.tw/wanker742126
/index.html
◦ SYMBOL is 6-byte, VALUE is 3-byte,
and FLAGS is 2-byte.

 STAB SYMBOL VALUE FLAGS
(100 entries) 6 3 2
LOC
1000 STAB RESB 1100
1000 SYMBOL EQU STAB +0
1006 VALUE EQU STAB +6
1009 FLAGS EQU STAB +9
 Use LDA VALUE, X to fetch the VALUE field form the
table entry indicated by the contents of register X.

 STAB SYMBOL VALUE FLAGS
(100 entries) 6 3 2
1000 STAB RESB 1100
ORG STAB
1000 SYMBOL RESB 6
1006 VALUE RESW 1
1009 FLAGS RESB 2
ORGSTAB+ 1100

 All terms used to specify the value of the
new symbol ---must have been defined
previously in the program.
...
BETA EQUALPHA
ALPHA RESW 1
...
Need 2 passes

 All symbols used to specify new location
counter value must have been previously
defined.
ORG ALPHA
BYTE1 RESB 1
BYTE2 RESB 1
BYTE3 RESB 1
ORG
ALPHA RESW 1
 Forward reference
ALPHA EQU BETA
BETA EQU DELTA
DELTA RESW 1
Need 3 passes

2.3.3 EXPRESSIONS
 Allow arithmetic expressions formed
◦ Using the operators +, -, ×, /.
◦ Division is usually defined to produce an integer
result.
◦ Expression may be constants, user-defined symbols,
or special terms.
106 1036 BUFEND EQU *
◦ Gives BUFEND a value that is the address of the next
byte after the buffer area.
 Absolute expressions or relative expressions
◦ A relative term or expression represents some value
(S+r), S: starting address, r: the relative value.

2.3.3 EXPRESSIONS
107 1000 MAXLEN EQU
BUFEND-BUFFER
◦ Both BUFEND and BUFFER are relative terms.
◦ The expression represents absolute value: the
difference between the two addresses.
◦ Loc =1000 (Hex)
◦ The value that is associated with the symbol that
appears in the source statement.
◦ BUFEND+BUFFER, 100-BUFFER,
3*BUFFER represent neither absolute values
nor locations.
 Symbol tables entries

2.3.4 PROGRAM BLOCKS
 The source program logically contained main,
subroutines, data areas.
◦ In a single block of object code.
 More flexible (Different blocks)
◦ Generate machine instructions (codes) and
data in a different order from the
corresponding source statements.
 Program blocks
◦ Refer to segments of code that are rearranged
within a single object program unit.
 Control sections
◦ Refer to segments of code that are translated
into independent object program units.

 Three blocks, Figure 2.11
◦ Default (USE), CDATA (USE CDATA),
CBLKS (USE CBLKS).
 Assembler directive USE
◦ Indicates which portions of the source
program blocks.
◦ At the beginning of the program, statements
are assumed to be part of the default block.
◦ Lines 92, 103, 123, 183, 208, 252.
 Each program block may contain several
separate segments.
◦ The assembler will rearrange these segments
to gather together the pieces of each block.

WRRE
C
Figure 2.11 Example of the Program with Multiple Program blocks

 Pass 1, Figure 2.12
◦ The block number is started form 0.
◦ A separate location counter for each program block.
◦ The location counter for a block is initialized to 0
when the block is first begun.
◦ Assign each block a starting address in the object
program (location 0).
◦ Labels, block name or block number, relative addr.
◦ Working table is generated
Block name Block number Address End Length
Default 0 0000 0065
0066(0~0065)
CDATA 1 0066 0070 000B
(0~000A)
CBLKS 2 0071 1070 1000
(0~0FFF)

Figure 2.12 Program from figure 2.11 with object code

 Pass 2, Figure 2.12
 The assembler needs the address for each symbol
relative to the start of the object program.
 Loc shows the relative address and block number.
 Notice that the value of the symbol MAXLEN (line 70)
is shown without a block number.
20 0006 0 LDA LENGTH
032060
0003(CDATA) +0066 =0069 =TA
using program-counter relative addressing
TA -(PC) =0069-0009 =0060 =disp

 Separation of the program into blocks.
◦ Because the large buffer (CBLKS) is moved to the
end of the object program.
◦ No longer need extended format, base register, simply
a LTORG statement.
◦ No need Modification records.
◦ Improve program readability.
 Figure 2.13
◦ Reflect the starting address of the block as well as the
relative location of the code within the block.
 Figure 2.14
◦ Loader simply loads the object code from each record
at the dictated.
◦ CDATA(1) & CBLKS(1) are not actually present in
OP.

Figure 2.13 Object Program corresponding to figure 2.11

Figure 2.14 Program blocks from fig 2.11 traced through the assembly
and loading processes

2.3.5 CONTROL SECTIONS & PROGRAM
LINKING
 Control section
◦ Handling of programs that consist of multiple
control sections.
◦ Each control section is a part of the program.
◦ Can be assembled, loaded and relocated
independently.
◦ Different control sections are most often used for
subroutines or other logical subdivisions of a
program.
◦ The programmer can assemble, load, and manipulate
each of these control sections separately.
◦ More Flexibility then the previous.
◦ Linking control sections together.

LINKING
 External references (external symbol references)
◦ Instructions in one control section might need to refer
to instructions or data located in another section.
 Figure 2.15, multiple control sections.
◦ Three sections, main COPY, RDREC, WRREC.
◦ Assembler directive CSECT.
◦ Assembler directives EXTDEF and EXTREF for
external symbols.
◦ The order of symbols is not significant.
COPY START 0
EXTDEF BUFFER,
BUFEND, LENGTH
EXTREF RDREC,
WRREC (symbol name)

Figure 2.15 Illustrations of Program linking and control sections

LINKING
 Figure 2.16, the generated object code.
15 0003 CLOOP +JSUB
RDREC 4B100000
160 0017 +STCH BUFFER,X
57900000
 The LOC of all control section is started form 0
 RDREC is an external reference.
 The assembler has no idea where the control section containing
RDREC will be loaded, so it cannot assemble the address.
 The proper address to be inserted at load time.
 Must use extended format instruction for external reference (M
records are needed).
190 0028 MAXLEN WORD BUFEND-
BUFFER
 An expression involving two external references.

Figure 2.16 Program from fig 2.15 with object code.

LINKING
◦ The loader will add to this data area with the
address of BUFEND and subtract from it the
address of BUFFER. (COPY and RDREC for
MAXLEN)
◦ Line 190 and 107, in 107, the symbols BUFEND
and BUFFER are defined in the same section.
◦ The assembler must remember in which control
section a symbol is defined.
◦ The assembler allows the same symbol to be used in
different control sections, lines 107 and 190.
 Figure 2.17, two new records.
◦ Defined record for EXTDEF, relative address.
◦ Refer record for EXTREF.

LINKING
 Modification record
◦ M
◦ Starting address of the field to be modified,
relative to the beginning of the control section
(Hex).
◦ Length of the field to be modified, in half-bytes.
◦ Modification flag(+ or -).
◦ External symbol.
M^000004^05+RDREC
M^000028^06+BUFEND
M^000028^06-BUFFER
 Use Figure 2.8 for program relocation.

Figure 2.17 Object Program corresponding to fig 2.15

2.4 ASSEMBLER DESIGN OPTIONS
2.4.1 TWO-PASS ASSEMBLER
 Most assemblers
◦ Processing the source program into two
passes.
◦ The internal tables and subroutines that
are used only during Pass 1.
◦ The SYMTAB, LITTAB, and OPTAB
are used by both passes.
 The main problems to assemble a program
in one pass involves forward references.

2.4.2 ONE-PASS ASSEMBLERS
 Eliminate forward references
◦ Data items are defined before they are
referenced.
◦ But, forward references to labels on
instructions cannot be eliminated as easily.
◦ Prohibit forward references to labels.
 Two types of one-pass assembler. (Fig. 2.18)
◦ One type produces object code directly in
memory for immediate execution.
◦ The other type produces the usual kind of
object program for later execution.

Figure 2.18 Sample Program for a one Pass assembler.

 Load-and-go one-pass assembler
◦ The assembler avoids the overhead of writing the
object program out and reading it back in.
◦ The object program is produced in memory, the
handling of forward references becomes less
difficult.
◦ Figure 2.19(a), shows the SYMTAB after scanning
line 40 of the program in Figure 2.18.
◦ Since RDREC was not yet defined, the instruction
was assembled with no value assigned as the
operand address (denote by ----).

Figure 2.19(a) Object code in memory and symbol table entries for the
program in fig 2.18 after scanning line 40

Figure 2.19(b) Object code in memory and symbol table entries for the
program in fig 2.18 after scanning line 150

 Load-and-go one-pass assembler
◦ RDREC was then entered into SYMTAB as an
undefined symbol, the address of the operand
field of the instruction (2013) was inserted.
◦ Figure 2.19(b), when the symbol ENDFIL was
defined (line 45), the assembler placed its
value in the SYMTAB entry; it then inserted
this value into the instruction operand field
(201C).
◦ At the end of the program, all symbols must
be defined without any * in SYMTAB.
◦ For a load-and-go assembler, the actual
address must be known at .

 Another one-pass assembler by generating OP
◦ Generate another Text record with correct
operand address.
◦ When the program is loaded, this address will be
inserted into the instruction by the action of the
loader.
◦ Figure 2.20, the operand addresses for the
instructions on lines 15, 30, and 35 have been
generated as 0000.
◦ When the definition of ENDFIL is encountered
on line 45, the third Text record is generated, the
value 2024 is to be loaded at location 201C.
◦ The loader completes forward references.

Figure 2.20 Object Program for one-pass assembler for Program in fig 2.18

 In this section, simple one-pass
assemblers handled absolute programs
(SIC example).

2.4.3 MULTI-PASS ASSEMBLERS
 Use EQU, any symbol used on the RHS be defined previously in the
source.
LOC Pass1 2 3
1000 LDA #0 1000 1000 1000
1003 ALPHA EQU BETA ???? ???? 1003
1003 BETA EQU DELTA ???? 1003 1003
1003 DELTA RESW1 1003 1003 1003
◦ Need 3 passes!
 Figure 2.21, multi-pass assembler
(a)

(b)

(c)

(d)

(e)

(f)

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