EP1200 Introduction to Computing Systems Engineering
Assembler Outlook: from Hack to a “real” computer
Outlook Outlook: from Hack to a “real” computer
The Hack CPU and computer HW is as simple as possible In reality design issues include • Memory hierarchy – memory access needs time and energy • Specific processors for specific tasks (graphics, floating point arithmetic) • Pipelining – Several consecutive instructions are processed at the same time, in different stages (e.g., instruction decode and computation) • Parallel processing – Instruction processed at several processors, or – Several instructions processed, if order does not matter • Communication inside the computer - how processors, memory, I/O devices interact – Buses and switches – a small network in itself Outlook: From Hack to a “real” computer
Contributed by Rajesh Kothandapani http://www.laynetworks.com Where we are at:
Human Abstract design abstract interface Software Thought Chapters 9, 12 hierarchy H.L. Language Compiler & abstract interface Chapters 10 - 11 Operating Sys. Virtual VM Translator abstract interface Machine Chapters 7 - 8 Assembly Language
Assembler
Chapter 6
abstract interface Computer Machine Architecture abstract interface Language Chapters 4 - 5 Hardware Gate Logic abstract interface Platform Chapters 1 - 3 Electrical Chips & Engineering Hardware Physics Logic Gates hierarchy Why care about assemblers?
Because …
Assemblers are the first step of the software hierarchy ladder
An assembler is a translator of a simple language – needs simple programming tools
Writing an assembler = practice for writing compilers For now, Assembler example ignore all details!
Source code (example) Target code // Computes 1+...+RAM[0] 0000000000010000 // And stored the sum in RAM[1] 1110111111001000 @i 0000000000010001 M=1 1110101010001000 @sum 0000000000010000 M=0 1111110000010000 (LOOP) assemble 0000000000000000 execute @i 1111010011010000 D=M 0000000000010010 @R0 1110001100000001 D=D-M 0000000000010000 @WRITE 1111110000010000 D;JGT 0000000000010001 ... // Etc. ... For now, Assembler example ignore all details! Source code (example) Target code
// Computes 1+...+RAM[0] 0000000000010000 // And stored the sum in RAM[1] 1110111111001000 @i 0000000000010001 M=1 1110101010001000 @sum 0000000000010000 M=0 1111110000010000 (LOOP) assemble 0000000000000000 execute @i 1111010011010000 D=M 0000000000010010 @R0 1110001100000001 D=D-M 0000000000010000 @WRITE 1111110000010000 D;JGT 0000000000010001 ... // Etc. ...
The program translation challenge Extract the program’s semantics from the source program, using the syntax rules of the source language Re-express the program’s semantics in the target language, using the syntax rules of the target language Assembler = simple translator Translates each assembly instruction into one binary machine instruction Handles symbols (e.g. i, sum, LOOP, …) – maintains a Symbol table
Translating / assembling A-instructions
Symbolic: @value // Where value is either a non-negative decimal number // or a symbol referring to such number.
value (v = 0 or 1)
Binary: 0 v v v v v v v v v v v v v v v
Translation to binary: If value is a non-negative decimal number, simple If value is a symbol (label or variable) get address from the symbol table Translating / assembling C-instructions
Symbolic: dest=comp;jump // Either the dest or jump fields may be empty. // If dest is empty, the "=" is ommitted; // If jump is empty, the ";" is omitted.
comp dest jump
Binary: 1 1 1 a c1 c2 c3 c4 c5 c6 d1 d2 d3 j1 j2 j3 Translating / assembling C-instructions
Translate the a,c,d,j bits: Use the definitions in the tables @R0 The overall assembler logic D=M @END D;JLE @counter For each (real) command M=D Parsing @SCREEN D=A break the command into its underlying fields (mnemonics) @x M=D Code generation (LOOP) A-instruction: replace the symbolic reference (if any) with @x A=M the corresponding memory address, using the symbol table M=-1 C-instruction: for each field in the instruction, generate the @x D=M corresponding binary code @32 Assemble the translated binary codes into a complete 16-bit D=D+A machine instruction @x M=D Write the 16-bit instruction to the output file @counter MD=M-1 @LOOP Note that comment lines and label declarations (pseudo D;JGT (END) commands) generate no code @END 0;JMP Typical symbolic Hack assembly code: (aka symbol resolution) Handling symbols @R0 D=M @END D;JLE Assembly programs can have many different symbols @counter M=D @SCREEN D=A Labels that mark destinations of goto commands (ROM) @x M=D • LOOP, END (LOOP) Pre-defined variables that mark special memory locations @x A=M (RAM) M=-1 • R0, SCREEN @x D=M User defined variables (RAM) @32 D=D+A • counter, x @x M=D @counter Symbols are maintained with the help of a symbol table. MD=M-1 @LOOP D;JGT (END) @END 0;JMP Handling symbols: symbol table
Source code (example) Symbol table // Computes 1+...+RAM[0] R0 0 // And stored the sum in RAM[1] R1 1 0 @i R2 2 1 M=1 // i = 1 ......
2 @sum R15 15 3 M=0 // sum = 0 SCREEN 16384 (LOOP) KBD 24576 Predefined RAM 4 @i // if i>RAM[0] goto WRITE SP 0 locations, filled in before the assembler 5 D=M LCL 1 process 6 @R0 ARG 2 7 D=D-M THIS 3 8 @WRITE THAT 4 9 D;JGT LOOP 4 10 @i // sum += i WRITE 18 Labels 11 D=M END 22 12 @sum i 16 13 M=D+M Variables sum 17 14 @i // i++ 15 M=M+1 16 @LOOP // goto LOOP 17 0;JMP This symbol table is generated by the (WRITE) 18 @sum assembler, and used to translate the 19 D=M symbolic code into binary code. 20 @R1 21 M=D // RAM[1] = the sum (END) 22 @END 23 0;JMP Typical symbolic Hack assembly code: (detailed) The assembly process @R0 D=M Initialization: @END D;JLE @counter • Create the symbol table, include pre-defined symbols M=D @SCREEN First pass: D=A @x • Go through the source code without generating any code. M=D (LOOP) • For each (LABEL) add the pair
Read Chapter 6 Assembler Set up a high level programming environment (suggested: Python)
Project: Complete the provided Assembler implementation Understand the provided incomplete code Add: Parsing and code generation for dest in C-instructions Add: Symbol table handling: address resolution for LABELs
Hand in Project 5 by April 7, 8:00.