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SDS: Allow LOAD command to accept a more flexible boot sequence

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The canonical boot sequence uses WIM 0,2 in location 5 and an initial load address in location 11 (and in the X register). This is limiting for devices like RAD that do not report an end-of-file, or for paper tape that report EOF too late and cause the WIM to fail. A simple load address in location 11, with bit 9 set to satisfy the BRX, doesn't do any word counting.  The BRX always branches back to location 5.

This change allows a true negative word count to be placed in location 11 to work with a WIM XXXXX,2 where XXXXX is one location past the region to be loaded by the boot sequence.  XXXXX plus the negative count in location 11 (buf[3]+buf[7]) initially points to the first location to be loaded, and the BRX will not branch when the word count is exhausted (register X=0) allowing the branch to the start address to be executed.

Old allowable boot sequence:
location  2 = WIM 12,2
location  3 = BRX 2
location  4 = LDX 11
location  5 = WIM 0,2
location  6 = SKS 21000
location  7 = BRX 5
location 10 = BRU Transfer Address
location 11 = Load address with bit 9 set to force BRX to branch

New allowable boot sequence:
location  2 = WIM 12,2
location  3 = BRX 2
location  4 = LDX 11
location  5 = WIM [end load address + 1],2
location  6 = SKS 21000
location  7 = BRX 5
location 10 = BRU Transfer Address
location 11 = Negative count to number of words to load
This commit is contained in:
Mark Emmer 2014-03-05 19:08:33 -06:00
parent ef99a17407
commit c256894b62
1 changed files with 2 additions and 2 deletions
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4
SDS/sds_sys.c
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@ -234,7 +234,7 @@ for (i = 0; i < 8; i++) { /* read boot */
if ((buf[0] != 023200012) || /* 2 = WIM 12,2 */
(buf[1] != 004100002) || /* 3 = BRX 2 */
(buf[2] != 007100011) || /* 4 = LDX 11 */
(buf[3] != 023200000) || /* 5 = WIM 0,2 */
(buf[3] & ~VA_MASK != 023200000) || /* 5 = WIM xxxxx,2 */
(buf[4] != 004021000) || /* 6 = SKS 21000 */
(buf[5] != 004100005)) /* 7 = BRX 5 */
return SCPE_FMT;
@ -242,7 +242,7 @@ for (i = 0; i < 8; i++) /* copy boot */
M[i + 2] = buf[i];
if (I_GETOP (buf[6]) == BRU)
P = buf[6] & VA_MASK;
for (i = buf[7] & VA_MASK; i <= VA_MASK; i++) { /* load data */
for (i = (buf[3]+buf[7]) & VA_MASK; i <= VA_MASK; i++) {/* load data */
if ((wd = get_word (fileref, &ldr)) < 0)
return SCPE_OK;
M[i] = wd;