OUT (0xFD)

The OUT instruction pops the top element from the stack, extracts the low-order byte, and writes that byte to the input/output window as character data.

The memory location at word-address 0x3FFD serves as a memory-mapped input/output port. When the microprogram writes to this address, the simulator extracts the low-order byte and sends it to the input/output window. The byte must represent a printable character, a backspace character or a new line character. Other byte values are ignored.

Note: the simulator word address space is 0x0000 to 0x3FFF. The I/O port address, 0x3FFD, corresponds to decimal -3. The microcode that interprets this instruction generates this address by performing the signed arithmetic (-1) + (-1) + (-1).

Interpreter Microcode

The microinstructions are listed in the order in which they are executed; not the order in which they are stored in the control store.

0x0fd  H=OPC=-1; goto 0x86
...
0x086  OPC=H+OPC; goto 0x87
0x087  MAR=H+OPC; goto 0x88
0x088  MDR=TOS; wr; goto 0x89
0x089  ALU=0; goto 0x8a
0x08a  SP=MAR=SP-1; rd; goto 0x8b
0x08b  ALU=0; goto 0x8c
0x08c  TOS=MDR; goto 0x2

Example Program

//---------------------------------------------
// Demonstrate the OUT instruction.
//
// 1. Clear Memory
// 2. Assemble this program.
// 3. Reset the computer.
// 4. Click the "Display Words" radio button
//    below the memory display.
// 5. Click the "Run" button.
//
// The text "OUT" will be displayed in the
// input/output window and the cursor will
// be at the beginning of the next line.
//---------------------------------------------
.main
    bipush 'O'
    out
    bipush 'U'
    out
    bipush 'T'
    out
    bipush 10   // new line
    out
    halt
.end-main