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

Concerned Application 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 Programs to sort, search, etc.

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

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

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

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

CHARACTERISTICS OF SIC

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

SIC Architecture - Registers 5 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 R

Instruction Formats - SIC 24 bit 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

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

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 (2 20 ) 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 Each special purpose

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

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 5 = = Character A

Data Formats Example Float 4.89 = * 2 3 (1027) =

Data Formats Example Float = * (1014) =

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

Instruction Formats – SIC/XE Format 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

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  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

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.

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