Subversion Repositories pdp1

loader as read in papertapeImages/20060517/musicPlayingProgram.bin

origin 7751

7751   73_0002  rpb             read paper binary

7752   32_7760  dio 7760        deposit IO in 7760

7753   10_7760  xct 7760        execute recently read instruction

7754   32_7776  dio 7776

7755   73_0002  rpb

7756   32_7777  dio 7777

7757   73_0002  rpb

7760   60_1411  jmp 1411        jump to 1411 (overwritten by 7752!)     -- holds DIO/write pointer

7761   21_7760  lac [7760]      load ac from wherever 7760 points       \

7762   40_7776  add 7776        add value from 7776 to AC                } *7776 += *7760

7763   24_7776  dac 7776        save AC there                           /

7764   44_7760  idx 7760        AC <- ++*7760

7765   52_7777  sas 7777        skip if AC=*7777

7766   60_7757  jmp 7757        jump back to 7757 if *7760!=*7777

7767   20_7776  lac 7776        AC <- *7776

7770   40_7777  add 7777        AC += *7777

7771   73_0002  rpb

7772   32_7776  dio 7776

7773   52_7776  sas 7776        skip if AC=*7776

7774   76_0400  hlt             halt if checksums didn't match?

7775   60_7751  jmp 7751        restart loader (executing the next tape instruction)

jumps to entry point 7751 after loading that code

from this we gather:

first word will be DIO origin, where origin is the first address the program is loaded to

this instruction will be stored at write pointer (7760), 7776 (checksum in progress) and origin

second word is deposited in 7777; this is DIO to the end address (first address NOT written)

third word etc is written at origin and added to checksum 7776 until write pointer=end address

we've now reached 7767

AC is loaded with end address plus checksum thus far, another word is read and deposited as checksum

if calculated checksum in AC does not match this newly read checksum at end, halt

otherwise, start loading all over again

this allows a block format, where each block is:

DIO start

DIO end

end-start data words

checksum

Finally, as the first word always executes, a jump instruction can start the program.

So, in papertapeImages/20060517/musicPlayingProgram.bin the program blocks are:

0003-0100 checksum 716574

0100-0200 checksum 623702       (assuming from here on 000000s are block boundaries)

0200-0300

0300-0400

0400-0500

0500-0546

jump to 0404, the entry point of the program