gistfile1.rb

typedef struct {
    VALUE *pc;			/* cfp[0] */
    VALUE *sp;			/* cfp[1] */
    VALUE *bp;			/* cfp[2] */
    rb_iseq_t *iseq;		/* cfp[3] */
    VALUE flag;			/* cfp[4] */
    VALUE self;			/* cfp[5]  / block[0] */
    VALUE klass;		/* cfp[6]  / block[1] */
    VALUE *lfp;			/* cfp[7]  / block[2] */
    VALUE *dfp;			/* cfp[8]  / block[3] */
    rb_iseq_t *block_iseq;	/* cfp[9]  / block[4] */
    VALUE proc;			/* cfp[10] / block[5] */
    const rb_method_entry_t *me;/* cfp[11] */
} rb_control_frame_t;

pc = position in bytecode of current instruction (maybe next instruction?)
sp = top of the operand stack
bp = bottom of the operand stack
iseq = instruction sequence currently being executed
flags = ... flags?
self = self
klass = probably the module that the method was found on (used for super)
lfp = locals frame pointer (i think), points to the start of where the local variables are stored
dfp = ?
block_iseq = pretty sure this is unused, there was just a patch to remove it
proc = unsure, perhaps the proc if the current frame is for a block
me = unsure, the method being executed?

dfp = block locals frame pointer?

/**
@c variable
@e get block local variable(which is pointed by idx and level).
level means nest level of block, and specify how above this variable.
@j level, idx Ç≈éwíËÇ≥ÇÍÇΩÉuÉçÉbÉNÉçÅ[ÉJÉãïœêîÇÃílÇÉXÉ^ÉbÉNÇ…íuÇ≠ÅB
level ÇÕÉuÉçÉbÉNÇÃÉlÉXÉgÉåÉxÉãÇ≈ÅAâΩíiè„Ç©Çé¶Ç∑ÅB
*/
DEFINE_INSN
getdynamic
(dindex_t idx, rb_num_t level)
()
(VALUE val)
{
rb_num_t i;
VALUE *dfp2 = GET_DFP();
for (i = 0; i < level; i++) {
dfp2 = GET_PREV_DFP(dfp2);
}
val = *(dfp2 - idx);
}

If I recall correctly...

flags = ... flags?

Bad name. Currently it represents frame type: VM_FRAME_MAGIC_*.

dfp = ?

dynamic frame pointer (= block locals frame pointer, as chad said)

def foo(a)
  b = 1
  # lfp = dfp, which has a and b
  foo do |c|
    d = 1
    # lfp has a and b
    # dfp has c and d
  end
end

block_iseq = pretty sure this is unused, there was just a patch to remove it

proc = unsure, perhaps the proc if the current frame is for a block

Both fields are actually used, via cast to type rb_block_t; The comment "cfp[8] / block[3]" means so.
They represents current given block.
block_iseq is accessed as rb_block_t.iseq, and represents iseq of given block.
proc is accessed as rb_block_t.proc, and represents a Proc object created for the block, if any.

me = unsure, the method being executed?

If the current frame is method, this field has the method info.