B5500: Added architecture description to head of b5500_cpu.c.

B5500/b5500_cpu.c

@@ -19,6 +19,77 @@
    IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
    CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 
+   The Burroughs 5500 was a unique machine, first introduced in 1961 as the
+   B5000. Later advanced to the B5500 (1964) adding disks and finally the B5700
+   (1971) adding solid state drum. It was the first computer to use the stack
+   as it's only means of accessing data. The machine could access at most
+   32k words of memory.
+
+   The machine used 48 bit numbers, all of which were considered to be floating
+   point numbers, integers were represented by a zero exponent. A word could
+   also be used to hold up to 8 6-bit characters.
+
+   The differences between the various models were minor. The 5500 added
+   the LLL, TUS, FBS and XRT instructions to improve performance of the OS. The
+   5700 added a core memory drum instead of spinning disk.
+
+   The 5500 series tagged memory to assist in controlling access.
+
+   The 5000 series did not have many programer accessible registers, all
+   operations were done on the stack. It had two modes of operation, character
+   and word mode.
+
+   A register 48 bits, held the top of stack.
+      AROF flag indicated whether A was full or not.
+   B register 48 bits, held the second element of the stack.
+      BROF flag indicated whether B was full or not.
+
+   S register 15 bits held a pointer to the top of stack in memory.
+   F register 15 bits held the Frame pointer.
+   R register 15 bits held a pointer to the per process procedures and
+        variables.
+   C register 15 bits together with the L register (2 bits) held the
+     pointer to the current executing sylable.
+
+   When in character mode the registers changed meaning a bit.
+
+   A held the Source word. GH two 3 bit registers held the character,bit
+      offset in the word.
+   B held the Destination word. KV two 3 bit registers held the
+      character and bit offset in the word.
+
+   The M register used to access memory held the address of the source
+     characters.
+   The S register held the address of the destination characters.
+   The R register held a count register refered to as TALLY.
+   The F register held the info need to return back to word mode.
+
+   The generic data word was: Flag = 0.
+
+                 11111111112222222222333333333344444444
+   0 1 2 345678 901234567890123456789012345678901234567
+  +-+-+-+------+---------------------------------------+
+  |F|M|E|Exp   | Mantissa                              |
+  |l|s|s|in    |                                       |
+  |a|i|i|octant|                                       |
+  |g|g|g|      |                                       |
+  | |n|n|      |                                       |
+  +-+-+-+------+---------------------------------------+
+
+  Also 8 6 bit characters could be used.
+
+  With the Flag bit 1 various data pointers could be constructed.
+
+                 11111111 112222222222333 333333344444444
+   0 1 2 345 678901234567 890123456789012 345678901234567
+  +-+-+-+---+------------+---------------+---------------+
+  |F|D|P|f  | Word count | F Field       | Address       |
+  |l|f|r|l  | R Field    |               |               |
+  |a|l|e|a  |            |               |               |
+  |g|a|s|g  |            |               |               |
+  | |g| |s  |            |               |               |
+  +-+-+-+---+------------+---------------+---------------+
+
 */
 
 #include "b5500_defs.h"
@@ -1978,7 +2049,6 @@ sim_instr(void)
         if (TROF == 0)
             next_prog();
 
-        /* TODO: Add support to detect IDLE loop */
 crf_loop:
         opcode = T & 077;
         field = (T >> 6) & 077;