jlh@loral.UUCP (Jim Vaxkiller) (11/04/86)
Is there a 'good' way to map C to hardware? Some specific examples are:
1) In my hardware both pointers and ints are 16 bits. I'm setting
up data structures to be traversed by microcode and whether
it's a pointer or data depends on where it is and how the microcode
got there. The way I'm doing it now is
typedef union {
int *ptr; /* might be a pointer */
int data; /* but then again, could be data */
} bsptr; /* be nice, it's bit slice pointer */
2) How about mapping bit fields into words? For example, I have
typedef struct {
cmd : 5; /* command word, bits 15-11 */
type : 1; /* xmit/receive, bit 10 */
subaddr : 5; /* subaddress, bits 9-5 */
words : 5; /* word count, bits 4-0 */
} data1553;
The individual fields MUST be in the bits specified. I could just
OR the data into the word but it would be 'cleaner' to use bit
fields.
3) There are instances where I have 2 eight bit datums and
I need to store them together in 1 16 bit word. Now I do a
*stash = (word) (hibyte << 8) + (lobyte & 0xf);
Is there a better way? Also, I have some 16 bit counters and need
to be sure the data is stored with the hi byte in bits 15-8.
4) I have some 32 bit counters that need to be stored with bits
31-16 in one word, then bits 15-0 in the next word. Now I have
a union, one entry being a long and the other 2 ints. Byte ordering
is very important here, see the previous question.
Probably the most important thing here is the results must be portable.
I'll probably write and debug most of it on a PC clone and VAX, then
download it into the actual hardware. The target microprocessor is
a 68010, which gives 3 widely differing architectures I need to work
in. Also, we may change compilers sometime down the road.
Please don't flame my syntax, I don't use typedefs and bit fields every
day and don't feel like digging out my K&R book.