1. Welcome to Systems Software
The purpose of this course is to provide background in fundamental types of system software, particularly assemblers, loaders, macro processors, and linkage editors.
The course uses a simplified instructional computer (SIC and SIC/XE) to illustrate machine level system software requirements.

2. Welcome to Systems Software
A major objective of the course will be for students to design and implement a working assembler for SIC/XE. (SIC extended)

3. Program Distinction Application programs Systems programs
Machine independent
Solves a specific problem
Systems programs
Machine dependent
Support computers operation
Compiler
Assembler
Linker
Loader OS

4. Application Programs
C/C++, Java, Perl, Python, Fortran, PL/1, LISP, Prolog, Pascal, C#, Ruby, Erlang, Clojure, etc.
Programs to sort, search, etc.

5. Systems Programs Compiler Assemblers Linkers Loaders OS
Translates application programs to intermediate code
Assemblers
Translates intermediate code to machine code
Linkers
Links the machine code modules into one
Loaders
Loads the machine code in memory OS
Controls the operation of the computer (provides an interface between the hardware and the user)

6. Example Assembler Program - SIC
LDA FIVE Load 5 into register A
STA ALPHA Store in location ALPHA
LDCH CHARZ Load character ‘Z’ into A
STCH C1 Store in C1 .
ALPHA RESW 1 one word variable
FIVE WORD 5 one word constant
CHARZ BYTE C’Z’ one byte constant
C1 RESB 1 one byte variable

7. Example Assembler Program – SIC/XE
LDA #5 Load 5 into register A
STA ALPHA Store in ALPHA
LDCH #90 Load character ‘Z’ into A
STCH C1 Store in C1 .
ALPHA RESW 1 one word variable
C1 RESB 1 one byte variable

8. SIC / SICXE Simple Instruction Computer
Simple Instruction Computer Extended
Designed to be similar to real computers
Is a virtual machine
Designed to avoid unnecessary detail

9. CHARACTERISTICS OF SIC
Bytes – 8 bits
Word – 3 bytes (24 bits)
Byte addressable
Words addressed by lowest byte
32767 (215) bytes of total memory

10. Characteristics of SIC (cont.) SIC Architecture - Registers
Each a full word
Each special purpose

11. Register Names and Usage - SIC
A Accumulator; used for arithmetic
X Index register; used for addressing
L Linkage resister; used for return address
PC 8 Program counter; address of next inst.
SW 9 Status word; variety of information including a condition code (CC)

12. Data Formats - SIC Integers – 24 bit binary 2’s complement
Characters – 8 bit ASCII
No floating point

13. Data Formats Example Integer Character A 01000001 R 01010010
Character A R

14. Instruction Formats - SIC
24 bit
LDA FIVE
00011A assuming FIVE is at address 11A
in binary
This is where we are going, it will all be explained
8 bit opcode bit addressing mode bit address

15. Addressing Modes - SIC Direct x = 0 Target address = address
Indexed x = 1 Target address + (X)
(X) is the contents of register X
LDA FIVE not indexed or
LDA FIVE,X indexed, like an array

16. Instruction Set – SIC see text for a complete list
Load and Store Registers
LDA, LDX, STA, STX, etc.
Integer Arithmetic (all involve register A)
Add, SUB, MUL, DIV
Compare
COMP – compares A with a word in memory
Sets the CC in the SW
Jump instructions
JLT, JEQ, JGT – based on the CC as set by COMP
Subroutine Linkage
JSUB – jumps to subroutine, places return address in L
RSUB – returns, using the address in L

17. Input/Output - SIC TD – test device is ready to send/receive data
CC of < means device is ready
CC of = means device is not ready
RD – read data, when the device is ready
WD – write data
Transfers 1 byte at a time to or from the rightmost 8 bits of register A.
Each device has a unique 8-bit code as an operand.

18. CHARACTERISTICS OF
SIC/XE

19. SIC/XE Architecture - Memory
Bytes – 8 bits
Word – 3 bytes (24 bits)
Byte addressable
Words addressed by lowest byte
1 meg (220) bytes of total memory (more memory leads to a change in instruction formats and addressing modes

20. SIC/XE Architecture - Registers
5 registers of SIC + 4 additional
Each a full word

21. Register Names and Usage – SIC and SIC/XE
A Accumulator; used for arithmetic
X Index register; used for addressing
L Linkage resister; used for return address
PC 8 Program counter; address of next inst.
SW 9 Status word; variety of information including a condition code (CC)

22. FOUR Additional Registers and their Usage SIC/XE
B Base register, used for addressing
S General register – no special use
T 5 General register – no special use
F 6 Floating-point accumulator (48 bits)

23. Data Formats – SIC/XE Integers – 24 bit binary 2’s complement
Characters – 8 bit ASCII
Floating point – 48 bit floating point

24. 1 bit sign 11 bit exponent 36 bit fraction
Data Formats – SIC/XE
24 bit integer
48 bit floating point
1 bit sign 11 bit exponent bit fraction

25. Floating Point Format SIC/XE
Fraction is a value between 0 and 1
The binary point is immediately before the high order bit which must be 1
The exponent is an unsigned binary number between 0 and 2047

26. Floating Point (cont) SIC/XE
Suppose the exponent is e and the fraction is f
The number is f * 2 (e+1024)
0 sign is positive
1 is negative
0 is all bits including sign are 0

27. Data Formats Example Integer Character A 01000001
5 =
-5 =
Character A

28. Data Formats Example Float 4.89 = .1001110001111010111000010100011110
* (1027) =

29. Data Formats Example Float -.000489 = .100000000011000000
* (1014) =

30. Instruction Formats – SIC/XE
Format bit (1 byte)
Format 2 – 16 bit (2 bytes)
ADDR T,A R2 <- (R2) + (R1)
9050
8 bit opcode
8 bit opcode bit R1 reg bit R2 reg

31. Instruction Formats – SIC/XE
Format bit (3 byte)
SUB N A <- (A) – (N)
1F2051
Opcode ni xbpe pc rel address
6 bit opcode n i x b p e bit displacemnt

32. Instruction Formats – SIC/XE
Format 4 – 32 bit (4 bytes)
+ADD SEC A <- (A) + (M..M+2)
1B100159
Opcode ni xbpe address
6 bit opcode n i x b p e bit address

33. Addressing Modes – SIC/XE Format 3/4 Instruction
Base relative b=1,p=0 TA = (B)+disp
0 <= disp <= 4095
disp is a n unsigned integer
PC relative b=0,p=1 TA = (PC)+disp
-2048 <= disp <= 2047
disp is a 2’s complement integer
Parenthesis indicates “the contents of”
if b=0, p=0 then disp is an absolute address

34. Addressing Modes – SIC/XE Format 3/4 Instruction (cont)
Any addressing mode can be combined with indexed addressing. i.e. if bit x is a 1 then (X) is added in the target address calculation.
Again, the parenthesis indicates contents of, i.e. (X) is the contents of register X

35. Addressing Modes – SIC/XE Format 3/4 Instruction the disp (cont)
Immediate addressing
i = 1, n = 0 the address itself is the operand, no memory reference
Indirect addressing
i = 0, n = 1 the word at the location is fetched as the address for the instruction
Simple addressing
i = n = 0 the target address is taken as the operand
e = 0 implies format 3, e = 1 implies format 4

36. Instruction Set – SIC/XE
Load and Store Registers
LDA, LDX, STA, STX, LDB, STB, RMO
Integer Arithmetic (all involve register A)
Add, SUB, MUL, DIV, ADDF, SUBF, MULF, DIVF, ADDR, SUBR, MULR, DIVR
Compare
COMP – compares A with a word in memory
Sets the CC in the SW
Jump instructions
JLT, JEQ, JGT – based on the CC as set by COMP
Subroutine Linkage
JSUB – jumps to subroutine, places return address in L
RSUB – returns, using the address in L

37. Input/Output – SIC/XE TD – test device is ready to send/receive data
CC of < means device is ready
CC of = means device is not ready
RD – read data, when the device is ready
WD – write data
Transfers 1 byte at a time to or from the rightmost 8 bits of register A.
Each device has a unique 8-bit code as an operand.
I/0 channels – SIO, TIO, HIO

38. Summary Addressing Modes
e = 0 – Format 3 instruction
e = 1 – Format 4 instruction

39. Summary Addressing Modes
Direct Addressing – b = p = 0
TA = disp
Relative Addressing –
b = 1, p = 0
TA = (B) + disp
b = 0, p = 1
TA = (PC) + disp

40. Summary Addressing Modes
Immediate
i = 1, n = 0
Target address itself is used as the operand value
Indirect
i = 0, n = 1
Value contained in the word is the address

41. Summary Addressing Modes
Simple
i = n = 0
TA is the location of the operand
SIC/XE instruction
i = n = 1
TA is determined by other bits