Qnice – An Elegant 16 Bit Processor, Hacker News

       

 0F 90  (MOVE 0x) , R0 0F 0100  (MOVE 0x) , R1  LOOP ADD R1, R0 3F 0100  (SUB 0x) , R1 FF8B  ABRA LOOP,! Z E 06 HALT        
             

       The QNICE assembler program shown on the left sums all values ​​from        0x (to 0x) .       

       This example is really simple – the main idea is to count backwards        from 0x 1100 to 0x and using the Z (zero) flag of the status        register R 16 as a control condition for the central loop.       

       Due to the possibility of making all branches and subroutine calls        conditional by prefixing them with the name of one of the eight status        register status bits the loop effectively consists of only two        instructions, a subtraction and an absolute branch (

ABRA ).              

       Prefixing the flag controlling the instruction with an exclamation mark        inverts the flag (without modifying the status register’s contents)        prior to testing thus the instruction

ABRA LOOP,! Z will        perform an absolute branch to the location labeled LOOP        if the zero flag is not set.             

       The code example below is a bit more complex and shows a subroutine        which performs a string comparison (like the C standard library        function strcmp). This function expects two pointers to the strings        to be compared in the registers R9 and R and will return the        result of the comparison in R8.       

       Since the routine needs some registers for temporary storage of        data is has to make sure that the contents of these registers are        somehow saved at the entry of the subroutine and restored just before        jumping back to the calling program.       

       In traditional architectures this would involve pushing the registers        to a stack (either explicitly, register by register, or controlled by        a bit mask). Since QNICE features a register bank for its registers        R0 to R7 saving and restoring the contents of these eight registers        just makes it necessary to increment the register bank pointer which        is made up by the eight upper bits of the status register R thus        giving the routine access to its “own” set of registers R0 to R7.        Before returning to the calling program the register bank pointer will        be decremented making sure that the original register contents will be        accessible again.       

       Incrementing the register bank pointer is done by performing the        instruction

ADD 0x , R
• , decrementing it by one is        done at the label STR $ _STRCMP_EXIT
• using the subtraction        
  SUB 0x 823, R

       

 ; ; ; STR $ STRCMP compares two strings ; ; R9: Pointer to the first string (S0), ; R : Pointer to the second string (S1), ; ; R8: negative if (S0 S1) ; ; The contents of R8 and R9 are being preserved during the run of ; this function ; ; STR $ STRCMP ADD 0x 0100, R ; Get a new register page                  MOVE R9, R0; Save arguments                  MOVE R 13, R1 STR $ _STRCMP_LOOP MOVE @ R0, R8; while  s1==s2   )                  MOVE @ R1   , R2                  SUB R8, R2                  RBRA STR $ _STRCMP_END,! Z                  MOVE @ R0   , R8; if  s1  ==0)                  RBRA STR $ _STRCMP_EXIT, Z; return 0;                  RBRA STR $ _STRCMP_LOOP, 1; end-of-while-loop STR $ _STRCMP_END MOVE @ - R1, R2; return  s1 - (- s2));                  SUB R2, R8 STR $ _STRCMP_EXIT SUB 0x 450, R ; Restore previous register page                  MOVE @R 18   , R ; and return to calling program part ;        
              
        A typical call to this routine would look like this:       
       
        
 MOVE QMON $ COMMAND, R9 MOVE QMON $ CMD_HALT, R 15 RSUB STR $ STRCMP, 1; Was a halt command issued?