basic assembler functions in system software.pptx

Source
Program Assembler
Object
Code
Executable
Code
Linker
1

2.1 Basic Assembler
Functions
3
 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.

7
 Figure shows an assembler language program
for SIC.
 The line numbers are for reference only.
 Lines beginning with “.” contain comments only.
 Reads records from input device (code F1)
 Copies them to output device (code 05)
 Subroutines (JSUB, RSUB)
 RDREC, WRREC
 Save link (L) register first before nested jump
 At the end of the file, writes EOF on the output device,
then RSUB to the operating system

2.1.1 A simple SIC
Assembler
16
 A forward reference
 10 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.

2.1.1 A simple SIC
Assembler
17
 Example of Instruction
Assemble
 Forward reference
 STCHBUFFER, X
(54)16 1 (001)2 (039)16
8 1 15
m
opcode x address
549039

2.1.1 A simple SIC
Assembler
 Forward reference
 Reference to a label that is defined later in the
program.
Loc Label OP Code Operand
1000 FIRST STL RETADR
1003 CLOOP JSUB RDREC
…
1012
…
…
…
…
J
…
…
CLOOP
…
1033 RETADR RESW 1
18

2.1.1 A simple SIC
Assembler
 The object program (OP) will be loaded
into memory for execution.
 Three types of records
 Header: program name, starting address,
length.
 Text: starting address, length, object code.
 End: address of first executable instruction.
19

2.1.1 A simple SIC
Assembler
20

2.1.1 A simple SIC
Assembler
 The symbol ^ is used to separate
fields.
 Figure 2.3
1E(H)=30(D)=16(D)+14(D)
21

2.1.1 A simple SIC
Assembler
22
 Assembler’s Functions
 Convert mnemonic operation codes to their
machine language equivalents
 STLto 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

20
2.1.1 A simple SIC
Assembler
 Write the OP (Fig. 2.3) and the assembly listing (Fig. 2.2).
 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.

2.1.2 Assembler Tables and Logic
24
 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.

25
 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).

26
 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.
COPY 1000
FIRST 1000
CLOOP 1003
ENDFIL 1015
EOF 1024
THREE 102D
ZERO
1030
RETADR
1033
LENGTH
1036
BUFFER
1039
RDREC 2039

27
 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]

else
if (found symbol==RSUB||
found symbol== …||
found symbol==…)
store 0 as operand address
else
store 0 as operand
address set error flag
assemble the object code
inst.
31

2.2 Machine-Dependent Assembler
Features
32
 Indirect addressing
 Adding the prefix @ to operand (line 70).
 Immediate operands
 Adding the prefix # to operand (lines 12, 25, 55,
133).
 Base relative addressing
 Assembler directive BASE (lines 12 and 13).
 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.

Features
36
 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)

Features
37
 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
38
 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

+OP, e=1
n=1, i=1,
@m, n=1, i=0,
#C, n=0,
i=1,
42
OPcode+3,
OPcode+2,
OPcode+1,
Extended
Simple
Indirect
Immediate
xbpe 2: PC-relative
4: base-relative
8: index (m,X)
1: extended

2.2.1 Instruction Formats &
Addressing Modes
43
3F2FEC
40 0017 J CLOOP
0006 - 001A = disp = -14
 Base (B), LDB #LENGTH, BASE
LENGTH
160 104E STCH BUFFER, X
57C003
55 0020 LDA #3 010003
133 103C +LDT #4096 75101000
TA-(B) = 0036 - (B) = disp = 0036-0033 =
0003
 Extended instruction
15 0006 CLOOP +JSUB RDREC
4B101036
 Immediate instruction

2.2.2 Program
Relocation
 Absolute program, relocatable
program
44

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
46

basic assembler functions in system software.pptx

More Related Content

Similar to basic assembler functions in system software.pptx

Recently uploaded

basic assembler functions in system software.pptx