EE 361 Homework 6 Solution

Problem A

There are two IO registers to transmit ASCII charcters to a computer terminal.

Transmitter Data Register: Its an 8-bit register with address 0xffff0000. ASCII character bytes are sent to the terminal from here. The Register has two states: "Ready" and "Not Ready". If the Register is Ready then an ASCII character can be loaded into it (in the low order byte of the register) by using the "sb" ("store byte) instruction (see pg. A-67). Then the Register will send the byte to the terminal. On the other hand, if the Register is Not Ready then it will ignore any "sb" instruction. Note that this Register is write only.
Transmitter Control Register: Its address is 0xffff0004 . This 8-bit register has status bits for the Transmitter Data Register. In particular, Bit 2 indicates when the Transmitter Data Register is Ready. If the Bit is equal to "1" then the Transmitter Data Register is Ready, and if the Bit is "0" then the Transmitter Data Register is Not Ready.

Solution:

putchar:
lui $t0,0xffff # Load upper half with $t0 all ones. If that doesn't work then you can try lui $t0,-1.
# Note that the lower half of $t0 is filled with zeros. Anyway, now $t0 = ffff0000
wait:
lbu $t1,0xc($t0) # Check if the transmitter data register is ready
andi $t1,$t1,4 # We're masking with 000....0100
beq $0,$t1,wait # If the bit2 = 0 then it's not ready

# At this point, bit 2 =1, so we proceed to output the char c.
sb $a0,0($t0) # Write the byte "c" (that's in $a0) to the transmitter data register.
# If that doesn't work, you can try sw $a0,0($t0)
jr $31