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The mix- function counterparts of the mixvm commands don’t
return any value, and are evaluated only for their side-effects
(possibly including informational messages to the standard output and/or
error stream). When writing your own Scheme functions to manipulate the
MIX virtual machine within mixguile (see Defining new functions), you’ll probably need Scheme functions returning the value
of the registers, memory cells and so on. Don’t worry: mixguile
also offers you such functions. For instance, to access the (numerical)
value of a register you can use mix-reg:
guile> (mix-reg 'I2) 0 guile>
Note that, unlike (mix-preg 'I2), the expression (mix-reg
'I2) in the above example evaluates to a Scheme number and does not
produce any side-effect:
guile> (number? (mix-reg 'I2)) #t guile> (number? (mix-preg 'I2)) rI2: + 00 00 (0000) #f guile>
In a similar fashion, you can access the memory contents using
(mix-cell), or the program counter using (mix-loc):
guile> (mix-cell 3000) 786957541 guile> (mix-loc) 3002 guile>
Other functions returning the contents of the virtual machine components
are mix-cmp and mix-over, which eval to the value of the
comparison flag and the overflow toggle respectively. For a complete
list of these additional functions, See mixguile.
In the next section, we’ll see a sample of using these functions to
extend mixguile’s functionality.