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 (cross) assembler for SIC and part of SIC/XE.

3. Concerned Application 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 Programs to sort, search, etc.

5. Systems Programs Compiler – 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

6. Assembler Program - SIC (Intermediate Code) LDAFIVELoad 5 into A STAALPHAStore in ALPHA LDCHCHARZLoad character ‘Z’ into A STCHC1Store in C1. ALPHARESW1one word variable FIVEWORD5one word constant CHARZBYTEC’Z’one byte constant C1RESB1one byte variable

7. Assembler Program – SIC/XE (Intermediate Code) LDA#5Load 5 into A STAALPHAStore in ALPHA LDCH#90Load character ‘Z’ into A STCHC1Store in C1. ALPHARESW1one word variable C1RESB1one byte variable

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

9. CHARACTERISTICS OF SIC

10. SIC Architecture - Memory Bytes – 8 bits Word – 3 bytes (24 bits) Byte addressable Words addressed by lowest byte 32767 (2 15 ) bytes of total memory

11. SIC Architecture - Registers 5 registers Each a full word Each special purpose

12. Register Names and Usage - SIC A 0 Accumulator; used for arithmetic X 1 Index register; used for addressing L 2 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)

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

14. Data Formats Example Integer 5 000000000000000000000101 -5 111111111111111111111011 Character A 01000001 R 01010010

15. Instruction Formats - SIC 24 bit 8 bit opcode 1 bit addressing mode 15 bit address

16. Addressing Modes - SIC Direct x = 0 Target address = address Indexed x = 1 Target address + (X) (X) is the contents of register X

17. Instruction Set - SIC 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

18. 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.

19. CHARACTERISTICS OF SIC/XE

20. SIC/XE Architecture - Memory Bytes – 8 bits Word – 3 bytes (24 bits) Byte addressable Words addressed by lowest byte 1 meg (2 20 ) bytes of total memory (more memory leads to a change in instruction formats and addressing modes

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

22. Register Names and Usage – SIC and SIC/XE A 0 Accumulator; used for arithmetic X 1 Index register; used for addressing L 2 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)

23. FOUR Additional Registers and their Usage SIC/XE B 3 Base register, used for addressing S 4 General register – no special use T 5 General register – no special use F 6 Floating-point accumulator (48 bits)

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

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

26. 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

27. 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

28. Data Formats Example Integer 5 = 000000000000000000000101 -5 = 111111111111111111111011 Character A 01000001

29. Data Formats Example Float 4.89 =.1001110001111010111000010100011110 10111000010100 * 2 3 (1027) =0 10000000011 100111000111101011 100001010001111010

30. Data Formats Example Float -. 000489 = 100000000011000000 111100000001111110 * 2 -10 (1014) =1 01111110110 100000000011000000 111100000001111110

31. Instruction Formats – SIC/XE Format 1 - 8 bit (1 byte) Format 2 – 16 bit (2 bytes) 8 bit opcode 8 bit opcode 4 bit R1 reg 4 bit R2 reg

32. Instruction Formats – SIC/XE Format 3 - 24 bit (3 byte) Format 4 – 32 bit (4 bytes) 6 bit opcode n i x b p e 20 bit address 6 bit opcode n i x b p e 12 bit displacemnt

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 (?) is the contents of register ? 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.

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