CSCI341 Lecture 27, ASCII &Unicode, Addressing Modes ASCII - - PowerPoint PPT Presentation

CSCI341

Lecture 27, ASCII &Unicode, Addressing Modes

ASCII

American Standard Code for Information Interchange One byte = One character How many possible characters?

COMMON INSTRUCTIONS

• lb (as signed)
• sb
• lbu (used in C compilers)

256 CHARACTERS

• How many symbols in Chinese?
• Arabic?
• Hindi?

UNICODE

• 16 bits to represent a character (how many are possible?)

COMMON INSTRUCTIONS

• lh (load halfword, as signed)
• lhu (more appropriate, common)
• sh (store halfword)

OPTIONAL READING

The Absolute Minimum Every Software Developer Absolutely,Positively Must Know About Unicode and Character Sets (No Excuses!) http://www.joelonsoftware.com/articles/Unicode.html

I-TYPE INSTRUCTIONS

• 16-bit “immediate” field
• What about longer numbers?
• What about longer addresses?

PROBLEM

Store the number 4,000,000 in a register. 0000 0000 0011 1101 0000 1001 0000 0000

LOAD UPPER IMMEDIATE

lui $s0, 61 first half: 0000 0000 0011 1101

• ri $s0, $s0, 2304

second half: 0000 1001 0000 0000

BIG CONSTANTS?

• Assembler must be aware, and break them down into pieces

at the instruction level.

• Uses register $at (reserved for assembler) to temporarily

store 32-bit constant

WHAT ABOUT ADDRESSES?

• I-Type instruction: address is 16 bits.
• Maximum address?
• Can’t a program consist of more instructions than that? (yes)

PC-RELATIVE ADDRESSING

• Assumption: conditional branches are within 215 away
• Branch address = PC + offset(in words)
• So for conditional branches, MIPS uses PC-relative addressing

BIGGER JUMPS (EG, TO FUNCTION LABELS)

• J-type instruction: 26-bits for addressing

ADDRESSING MODES

Immediate:

• perand is constant

Register:

• perand is value in a

register Base/Displacement:

• perand is register value

plus constant PC-Relative:

• perand PC + constant

Pseudodirect: Think: J-Type

HOMEWORK

• Reading: 2.10 & 2.11
• Finish Project 5, “Absolutely!”

MIPS vs. E-Days... hmm...