Registers

Registers are internal local variables that are used to hold interim 
results and return or pass information. There are 256 32-bit 
noninitialized registers that are private to every VIDEOPMI caller. 
Registers should be assigned their values prior to being used. They can be 
used as a source in port OUT commands and memory WRITE commands or as a 
destination in the port IN commands and memory READ commands. Size of 
these operations is determined by the size of the PMI command. A set of 
32-bit operations on registers is also defined in the syntax. When used in 
block I/O commands, the register index should be the same as that of the 
port index to which it corresponds. Register r0 is used as the return 
value register from functions that have a return value. Register r0 is 
also used to provide the bank number to the SetBank function. 

Care should be taken in assigning registers in sections that contain 
function calls. Because all of the internal and imported PMI functions 
have addressability to the same set of registers, they will overwrite 
values set before function calls. 

<register>  = r<'0'..'255'>