Printing a number contained in a register

Question

I'm learning MMIX so I tried making a simple program to add one to itself and print the result. Unfortunately it doesn't print anything. Here is my program:

n    IS $4 
y    IS $3
t    IS $255
     LOC #100
Main SET n,1 %let n = 1
     ADD y,n,1 %add 1 to n and store the result in y
     LDA t,y 
     TRAP 0,Fputs,StdOut
     TRAP 0,Halt,0

What am I doing wrong?

Answer 1

I ended up figuring it out after seeing the code here. I had to first create a byte, then store the value of the register into the byte. Then by printing out that byte, I get the result of ADD y,n,1.

Answer 2

The link in Robert's own response is broken. Also the explanation is unsatisfactory.

The main issue is there is no printf in MMIX assembly. So you can't just print a number directly. It needs to be converted to a string for Fputs to work.

Once you know this the solution is easy. The challenge is to code it in MMIX. The program below handles one unsigned number.

// printnum.mms
// run with MMIX simulator or visual debugger: https://mmix.cs.hm.edu

n        IS $4
y        IS $3
t        IS $255

// a register for extracting a digit
digit    IS $5
// a 16-byte buffer for the converted string
buf      OCTA 0

         LOC #100
Main     SET n,1 %let n = 1
         ADD y,n,1 %add 1 to n and store the result in y

// convert y to ascii digits and store in buf

         GETA t,buf+16
// divide and set digit to the remainder register rR
1H       DIV y,y,10
         GET digit,rR
// convert digit to ascii character
         INCL digit,'0'
// fill buffer from the end
         SUB t,t,1
         STBU digit,t,0
// loop back to 1H for more digits
         PBNZ y,1B

// print the converted string
// this works because string offset is already in register $255
         TRAP 0,Fputs,StdOut

         TRAP 0,Halt,0