IJVM exercise, swapping 2 elements in a 4 elements stack

Question

i'm trying to create a microprogrammed function that, given 4 values in the stack, swaps the 3rd and the fourth. I called it "mswap", this is the microcode so far:

mswap1 SP = SP - 1 mswap2 SP = SP - 1
mswap3 MAR = SP - 1
mswap4 rd mswap5 MAR = H
mswap6 wr mswap7 MAR = SP
mswap8 rd mswap9 MAR = SP - 1 mswap10 wr
mswap11 MAR = H mswap12 rd mswap13 MAR = SP mswap14 wr mswap15 SP = SP + 1 mswap16 SP = SP + 1; goto Main1

And this is an example program:

.main start: BIPUSH 0x39 BIPUSH 0x30 BIPUSH 0x36 BIPUSH 0x35 MSWAP OUT OUT OUT OUT HALT .end-main

It should give an output of 5690 Sad truth, the output is just 56 it's ike i "missed" two pile elements.. Is this due to excessive use of SP = SP -1? It worked just fine when writing a similar function to swap the 2nd and 3rd element

Answer 1

mswap5 is using H, but H has not been set. Did you mean to set it in an earlier instruction?

Only two reads and two writes are needed.

SP does not need to be changed in order to access into the stack. Use H, then you will not have to adjust SP at the end of the routine.