Below is the MIPS code for two subroutines: readchar: la $t0, 0xffff0000 lw $t1,

Question

Below is the MIPS code for two subroutines:
readchar:
la $t0, 0xffff0000
lw $t1, 0($t0)
andi $t2, $t1, 1
beq $t2, $zero, readchar
lb $v0, 4(($t0))
jr $ra
writechar:
la $t0, 0xffff0000
lw $t1, 8($t0)
andi $t2, $t1, 1
beq $t2, $zero, writechar
sb $a0, 12($t0)
jr $ra

Explain in words what each line of the two subroutines is doing with respect to the use of the command register and a control register and the appropriate steps for each of these two operations. Note I do not simply want the literal meaning of each MIPS. I would like you to explain the detail of how MIPS implements memory mapped I/O

Explanation / Answer

Initially, the address Oxffffoooo is loaded into the register tO. Then the value stored in tO is loaded into the register t1. After loading the values, immediate AND opeartion is performed between the content in the register t1 and constant 1and the value is stored in register t2. The next instruction is the branch instruction. Here the value in t2 register is compared other register with constant value of zero and if the values in both are the same,then the read operation is carried out by the control register. The previous operation of comparing the values is done by command register. If the values are not the same,read operation will not be performed. Now the values obtained as a result of the above functions in stored in vO. The jump register is used to jump to a particular register address.

Now the read operation is over and the write operation is performed. Here initially,the address Oxffffoooo is loaded into register tO. Then the value in tO is loaded into register t1. Immediate AND operation is performed with the value stored in t1 and constant 1 and the value is stored in register t2. Now for branching,if the value stored in register t2 is equal to value stored in zero register,the write operation is enabled. At last the values are stored to register aO and jump register is performed.

Here the most important thing is during branching operation,only if the given condition is satisfied, the specified operation is performed in the next stage.Or else the branching operation goes to the end of the program or instruction.

All the branching operations are controlled by the control register while the comparisons are done by command register. Based on memory mapped io,it can be explained that during the loading and storing of the values into different registers,automatically the memory allocates space for the values during input and output values.

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