Today’s class

1. Today’s class • More assembly language programming Computer Architecture I - Class 2

2. Data transfer instructions • lw – load word from memory into a register (note that the address must be a word address, divisible by 4) • lb – load byte • sw – store word into memory from a register (address must be for a word) • sb – store byte Computer Architecture I - Class 2

3. Addition (signed) • add regdest,regsrc1,regsrc2 • Performs regdest = regsrc1 + regsrc2 • Uses register addressing (no memory reference) • addi regdest,regsrc,const • Performs regdest = regsrc + const • Add immediate, because const is part of instruction (no memory reference) • add regdest,regsrc,const • Will generate an addi instruction by the assembler • add regdest, const • Will generate addi regdest,regdest,const Computer Architecture I - Class 2

4. Addition (unsigned) • addu regdest,regsrc1,regsrc2 • Performs regdest = regsrc1 + regsrc2 • addiu regdest,regsrc,const • Performs regdest = regsrc + const • addu regdest,regsrc,const • Will generate an addiu instruction by the assembler • addu regdest, const • Will generate addiu regdest,regdest,const Computer Architecture I - Class 2

5. Subtraction • sub regdest,regsrc1,regsrc2 • Performs regdest = regsrc1 - regsrc2 • This is a signed subtraction • subu regdest,regsrc1,regsrc2 • Performs regdest = regsrc1 - regsrc2 • This is an unsigned subtraction • No subi instruction • Can do an addi with the negative of what you want to subtract Computer Architecture I - Class 2

6. Multiplication • Result of multiplying two 32-bit numbers can be 64 bits big • Have two special registers called lo and hi to hold the two 32-bit parts of the result • mult regsrc1,regsrc2 will put the low 32 bits of the product in lo and the high 32 bits in hi • Use mflo regdest (move from lo) to get the low 32 bits into a register • Use mfhi regdest (move from hi) to get the high 32 bits into a register • The pseudo-instruction mul regdest,regsrc1,regsrc2 will get the low 32 bits of the product into regdest Computer Architecture I - Class 2

7. Division • div regsrc1,regsrc2 will put the integer quotient of regsrc1/regsrc2 in lo and the remainder in hi • The pseudo-instruction div regdest,regsrc1,regsrc2 will get the integer quotient into regdest Computer Architecture I - Class 2

8. Example program • Program temperature.asm converts Celsius temperatures to Fahrenheit • Study it and demo it Computer Architecture I - Class 2

9. In-class exercise • Modify the temperature conversion program so it converts from Fahrenheit to Celsius instead. Computer Architecture I - Class 2

10. Unconditional branching • j addr – jumps to the indicated address and continues execution from there • jr reg – jumps to the address stored in the specified register and continues execution from there Computer Architecture I - Class 2

11. Conditional branching • beq regsrc1,regsrc2,addr – branch to addr if regsrc1 equals regsrc2 • beqz reg,addr – branch to addr if reg equals 0 • bne regsrc1,regsrc2,addr – branch to addr if regsrc1 does not equal regsrc2 • blt regsrc1,regsrc2,addr – branch to addr if regsrc1 is less than regsrc2 • The assembler will let the second operand be a constant instead of a register if desired • There are many more – see Appendix B (page 146) of Waldron Computer Architecture I - Class 2

12. Example programs • Program length.asm prints out the length of a character string • Program replace.asm replaces lower case ‘a’ with upper case ‘A’ in a string • Study them and demo them Computer Architecture I - Class 2

13. In-class exercise • Write a program to count how many times the letter ‘e’ appears in a string. Computer Architecture I - Class 2