1 / 14

main: Loop: sw $0, DEFN($0) jal getline la $s0, inBUF

What Does an Assembler Do ?. main: Loop: sw $0, DEFN($0) jal getline la $s0, inBUF lb $t0, ($s0) lb $t1, DOLLAR($0) beq $t0, $t1, QUIT lb $t1, L($0) bne $t0, $t1, get_tok

lorin
Download Presentation

main: Loop: sw $0, DEFN($0) jal getline la $s0, inBUF

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. What Does an Assembler Do ? main: Loop: sw $0, DEFN($0) jal getline la $s0, inBUF lb $t0, ($s0) lb $t1, DOLLAR($0) beq $t0, $t1, QUIT lb $t1, L($0) bne $t0, $t1, get_tok li $s0, 1 sw $s0, DEFN($0) get_tok:

  2. What Does an Assembler Do ? main: loop: sw $0, DEFN($0) jal getline la $s0, inBUF lb $t0, ($s0) lb $t1, DOLLAR($0) beq $t0, $t1, QUIT lb $t1, L($0) bne $t0, $t1, get_tok li $s0, 1 sw $s0, DEFN($0) get_tok: Read Each Input Line For each input line:

  3. What Does an Assembler Do ? main: loop: sw $0, DEFN($0) jal getline la $s0, inBUF lb $t0, ($s0) lb $t1, DOLLAR($0) beq $t0, $t1, QUIT lb $t1, L($0) bne $t0, $t1, get_tok li $s0, 1 sw $s0, DEFN($0) get_tok: Read Each Input Line For each input line: SAVE Labels and LOCs

  4. What Does an Assembler Do ? main: loop: sw $0, DEFN($0) jal getline la $s0, inBUF lb $t0, ($s0) lb $t1, DOLLAR($0) beq $t0, $t1, QUIT lb $t1, L($0) bne $t0, $t1, get_tok li $s0, 1 sw $s0, DEFN($0) get_tok: Read Each Input Line For each input line: SAVE Labels and LOCs MAP op to machine code

  5. What Does an Assembler Do ? main: loop: sw $0, DEFN($0) jal getline la $s0, inBUF lb $t0, ($s0) lb $t1, DOLLAR($0) beq $t0, $t1, QUIT lb $t1, L($0) bne $t0, $t1, get_tok li $s0, 1 sw $s0, DEFN($0) get_tok: Read Each Input Line For each input line: SAVE Labels and LOCs MAP op to machine code OPERANDS

  6. What Does an Assembler Do ? main: loop: sw $0, DEFN($0) jal getline la $s0, inBUF lb $t0, ($s0) lb $t1, DOLLAR($0) beq $t0, $t1, QUIT lb $t1, L($0) bne $t0, $t1, get_tok li $s0, 1 sw $s0, DEFN($0) get_tok: Read Each Input Line For each input line: SCAN each line to tokens SAVE Labels and LOCs MAP op to machine code OPERANDS => PARSE tokens

  7. Scanner • Scan each input line • Get a char and its type (HW2) • If current char and its preceding one is related • Keep collecting as a single token • Otherwise, • Save as a new token • Example • THISLOOP: LWU R2,63 # • Token Token Type • ---------------------------- ---------------- • THISLOOP 2 • : 4 • LWU 2 • R2 2 • , 4 • 63 1 • # 5

  8. Automata • Given a state, no changes until an external stimulus • Light switches • Traffic lights • A system with a memory • How to relate one char to its predecessor ? • Repetitive actions • Get a char, search for its type • New token • Collect a char into a token string • Complete assembly of a token • State Diagrams

  9. State Transition Diagram Q0 T=5 TOKEN=curChar, TokSpace=7 (ACT 2) Q1 T=1 curChar =Get next char, T=ChType(curChar) (ACT 1) T=3,4 T=6 T=2 curChar =Get next char, T=ChType(curChar) (ACT 1) Q3 Q4 Q2 Save TOKEN into TokArray (ACT 4) Q5 Return Q0 Save TOKEN into TokArray (ACT 4) T=1,2 T <> 1,2 T=1 T <> 1 Save TOKEN into TokArray (ACT 4) Q9 Q6 Q7 Q8 TOKEN=TOKEN+curChar TokSpace=TokSpace-1 (ACT 3) TOKEN=TOKEN+curChar TokSpace=TokSpace-1 (ACT 3) Q1 Q1

  10. Map Operator

  11. Operand • Registers • Replace by Reg Number • Literal • Easy • Variable (Label) • Backward Reference – simple • Forward Reference ==== PROBLEM main: loop: sw $0, DEFN($0) jal getline la $s0, inBUF lb $t0, ($s0) lb $t1, DOLLAR($0) beq $t0, $t1, QUIT lb $t1, L($0) bne $t0, $t1, get_tok li $s0, 1 sw $s0, DEFN($0) get_tok:

  12. 2-pass Assembler • Pass 1: construct a symbol table • Read a line and scan it • Check for label: save in Symbol Table with LOC as value • Check op field: if machine op, increment LOC • If neither, an error • Pass 2: • Reset LOC • Read a line and scan it • label: skip if found in symbol table • machine op: find op-code and default values, increment LOC • Operand: check expression • Assemble and output

  13. 1-pass Assembler • Read a line and scan • Label ? • Enter into symbol table with LOC as defined value • Operator • If machine op • Look up machine code and save • Increment LOC • If pseudo op • Process as necessary • Otherwise • error • Operand • Register – get reg # • Expression – evaluate • Assemble

  14. Forward Reference Symbol value status FAN exit 1000 100 exit 1010 000 exit 1020 000 exit 1030 001 • Loc instr assembled • j exit 0b ff ff ff • beq $t1, $t0, exit • 11 28 1000 • blez $s0, exit • 1a 00 1010 • 1030 exit: jr $ra 03 e0 FF • Forward reference • Symbol not found • Assemble with FFFFx as offset • save the symbol with LOC as it value • Symbol found, but another forward ref. • Assemble with value in symTab as offset • Save LOC as value in symTab • Symbol found, new defintion • Assemble with offsets by tracing back pointers • Save LOC as value in symTab

More Related