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.

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)

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

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

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)

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

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

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

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

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

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)

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

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

Instruction Formats - SIC 24 bit LDA FIVE 00011A assuming FIVE is at address 11A 0000 0000 0000 0001 0001 1010 in binary This is where we are going, it will all be explained 8 bit opcode 1 bit addressing mode 15 bit address

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

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

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.

CHARACTERISTICS OF SIC/XE

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

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

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)

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)

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

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 36 bit fraction

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

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

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

Data Formats Example Float 4.89 = .1001110001111010111000010100011110 10111000010100 * 23 (1027) =0 10000000011 100111000111101011 100001010001111010

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

Instruction Formats – SIC/XE Format 1 - 8 bit (1 byte) Format 2 – 16 bit (2 bytes) ADDR T,A R2 <- (R2) + (R1) 9050 8 bit opcode 8 bit opcode 4 bit R1 reg 4 bit R2 reg

Instruction Formats – SIC/XE Format 3 - 24 bit (3 byte) SUB N A <- (A) – (N) 1F2051 0001 1111 0010 0000 0101 0001 Opcode ni xbpe pc rel address 6 bit opcode n i x b p e 12 bit displacemnt

Instruction Formats – SIC/XE Format 4 – 32 bit (4 bytes) +ADD SEC A <- (A) + (M..M+2) 1B100159 0001 1011 0001 0000 0000 0001 0101 1001 Opcode ni xbpe address 6 bit opcode n i x b p e 20 bit address

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

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

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

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

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

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

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

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

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