bytes, you still have 224 bytes available for your own programs.

Keyboard Systems. Adding a hex keyboard would make your Elf microcomputer even easier to use, with 16 keys labelled 0 through F, you would have to press only two keys for each byte you want to store in memory. In the second article, we described a circuit for monitoring the status of 16 switches or keys. (See POPULAR ELECTRONICS, Sept. 1976, page 38, Fig.3). If you add this circuit and a 16-key hex keyboard, you can use Program 2--EHOPS-256 (Elf Hex OPerating System for 256-byte memory). This program requires 74 bytes of RAM so you still have 182 bytes left for your own programs. You can also use the hex keyboard subroutine as part of your program if desired.

       PROGRAM 1--ETOPS-256

 0000   F8  20  A1   R1.0 = work
   03   E1           X=1
   04   6C  64  21   D = toggles
   07   3F  07       Wait for IN on
   09   37  09       Wait for IN off
   0B   32  1D       M(1D) if D=00
   0D   F6  33  11   M(11) if D=01
   10   7B           Q=1
   11   6C  A1       R1.0 = toggles
   13   3F  13       Wait for IN on
   15   37  15       Wait for IN off
   17   39  1A       M(1A) if Q=0
   19   6C           M1 = toggles
   1A   64           Show M1, R1 + 1
   1B   30  13       Repeat M(13)
   1D   6C  A3       R3.0 = toggles
   1F   D3           P=3
   20   00           Work area
   21   User programs from
          M(21) to M(FF)
 

After loading EHOPS in memory, you can use it as follows. To load a byte into any memory location from the hex keyboard, set the toggles to 02 and flip the RUN switch up. The 02 toggles tell EHOPS that you want to store bytes in memory. On the hex keyboard, press the most significant digit of a memory address followed by the least-significant digit. This address byte will be displayed and tells EHOPS where you want to start loading bytes in memory. You can now load a sequence of bytes into memory via the hex keyboard. Just press the two digits (most significant first) of each byte you want to load and they will be stored sequentially in memory starting at the selected location.

       PROGRAM 2--EHOPS-256
 0000   F8  FF  A2   R2.0 = work
   03   F8  23  A5   R5.0 = BSUB
   06   F8  33  A6   R6.0 = HSUB
   09   F8  0D  A3   R3.0 = M(0D)
   0C   D3           P=3
   0D   D5  A1       BSUB, R1.0=D
   0F   6C           D, M2 = toggles
   10   3A  14       M(14) if D != 00 (Note: != means 'not equal')
   12   81  A3       R3.0 = R1.0
   14   F6  3B  1C   M(1C) if D=02
   17   D5  E1       BSUB, X=1
   19   64           Show M1, R1+1
   1A   30  17       Repeat M(17)
   1C   D5  E1       BSUB, X=1
   1E   51  64       M1=D, show M1, R1+1
   20   30  1C       Repeat M(1C)
   22   D3           P=3 (return)
  BSUB
   23   D6           HSUB
   24   FE  FE       D left x 2
   26   FE  FE       D left x 2
   28   A0  D6       R1=D, HSUB
   2A   80  F1  52   M2=R1 or M2
   2D   64  22       Show M2
   2F   30  22       Go to M(22)
   31   F0  D5       D=M2, P=5
  HSUB
   33   E2  FC  01   X=2, D+1
   36   FA  0F  52   M2=D and 0F
   39   62  22       Select key M2
   3B   3D  33       M(33) if key off
   3D   7B  F8  09   Q=1, D=09
   40   B4           R4.1=09
   41   24  94       R4-1
   43   3A  41       M(41) if R4.1 != 00
   45   7A           Q=0
   46   35  46       Wait for key off
   48   30  31       Go to M(31)

To examine any memory location (without changing its contents), set the toggles to 01 before you flip the RUN switch up. Using the hex keyboard, enter the one-byte starting address of the sequence of memory locations you want to examine. Press any hex key twice to step through memory and display the stored bytes.

To run a program you've loaded using EHOPS, set the toggles to 00 before flipping the RUN switch up. Using the hex keyboard, enter the one-byte starting address of your program. It will begin running with R3 as the program counter.

          PROGRAM 3

 0050  F8  FF  A1       R1.0 = work
   53  F8  00  51       M1=00
   56  E1  64  21       Show M1
   59  F0  FC  01  51   M1+1
   5D  F8  10  B2       R2.1 = delay
   60  22               R2-1
   61  92  3A  60       M(60) if R2.1 != 00
   64  30  56           Repeat M(56)

EHOPS controls the hex keyboard with two subroutines called BSUB and HSUB. BSUB calls HSUB by changing the program counter to R6 with a D6 instruction. HSUB continuously scans all 16 hex keyswitches until one is pressed. It provides a switch debounce delay and waits until the key has been released. It then returns control to BSUB with the value of the pressed key in the least-significant digit of the byte in D and M2.

BSUB is called by changing the program counter to R5 with a D5 instruction. It waits until two hex keys have been pressed before returning control to the calling program with the values of the two keys in the two digits of the byte in D and M2. The most-significant digit represents the first key pressed. Any program you write with R3 as the program counter can call BSUB to obtain a byte from the hex keyboard. If you drive a speaker with the Q lines as described in the September article, you will hear an audible click each time a hex key is pressed.

Program 3 can be loaded and run us-[65}