Program Loader

This program is the "operating system" of the computer. It takes hex character input from a terminal and converts it to 16-bit instruction words which are loaded into RAM starting at 0x80A. Upon receiving a ctrl-c, it jumps to 0x80A and begins to execute the loaded instructions. It needs upper case characters for A-F. Note that since the character input is 8-bit, and the path through the ALU is 12 bit, there is no easy way to get the highest order nybble of a 16-bit instruction word from the UART into memory. I solved this problem by making a table that contains the high order nybbles. A table entry is selected by the first character of each instruction word, and subsequent characters are converted to binary and placed into the word using the ALU. Later, I built a byte-switcher port which can more directly get low-order data into the high-order accumulator bits, and from there into memory. The instruction table had a gap in it due to the ASCII values of the characters, so I put some constants in the gap to save space.

The instruction set has does not have an instruction for indexed memory access, which is used to access a table. However, indexing can be done indirectly with the following procedure. Two words of RAM are set aside, the first word for the indexed instruction, the second for a return instruction (JPI or JMP). The index value is loaded into the accumulator, and the instruction to be indexed is added (using the ADD instruction). Even though the ALU is only 12 bits wide, the procedure works because the upper 4 bits of the accumulator will always contain the upper 4 bits of the most recently accessed value in memory. The indexed instruction is then placed in the RAM location set aside for it, then a JMP to that location is performed. After the instruction is finished, the return jump is performed. The program loader uses this technique. The base instructions are stored in the table gap in ROM at 0x07C and following, and there are RAM locations set aside for an indexed instruction and following return jump at 0x803 and 0x804.

There is a little patch of code at the end to allow the UART to divide its clock by 16. This helped character flow by improving start bit detection and bit framing. The program code can be entered at 0x027 or 0x08E. The program is in ROM at 0x027 because above it are some small test programs like the port reflector and factor routine (see Programming). In its finished form, ROM has a JMP 0xC01 at the start. This instruction executes whatever instruction is on the lower DIP switches on the memory I/O board. In the picture, you can see the instruction word B08E there in the switches, which is JMP 08E.