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As the cathode-ray tube warms up, the screen displays an array of perfectly formed—but totally random—ASCII characters. This is as we expect. Semiconductor memory loses its contents when the power is off and begins in a random and unpredictable state when it first gets power. Likewise, all the RAM that we've constructed for the microprocessor contains random bytes. The microprocessor begins executing these random bytes as if they were machine code. This won't cause anything bad to happen—the computer won't blow up, for instance—but it won't be very productive either.

What we're missing here is software. When a microprocessor is first turned on or reset, it begins executing machine code at a particular memory address. In the case of the Intel 8080, that address is 0000h. In a properly designed computer, that memory address should contain a machine-code instruction (most likely the first of many) when the computer is turned on.

How does that machine-code instruction get there? The process of getting software into a newly designed computer is possibly one of the most confusing aspects of the project. One way to do it is with a control panel similar to the one in Chapter 16 used for writing bytes into random access memory and later reading them:

Unlike the earlier control panel, this one has a switch labeled Reset. The Reset switch is connected to the Reset input of the microprocessor. As long as that switch is on, the microprocessor doesn't do anything. When you turn off the switch, the microprocessor begins executing machine code.

To use this control panel, you turn the Reset switch on to reset the microprocessor and to stop it from executing machine code. You turn on the Takeover switch to take over the address signals and data signals on the bus. At this time, you can use the switches labeled A0 through A15 to specify a 16-bit memory address. The lightbulbs labeled D0 through D7 show you the 8-bit contents of that memory address. To write a new byte into that address, you set the byte up on switches D0 through D7 and flip the Write switch on and then off again. After you're finished inserting bytes into memory, turn the Takeover switch off and the Reset switch off, and the microprocessor will execute the program.

This is how you enter your first machine-code programs into a computer that you've just built from scratch. That it's laborious goes without saying. That you will make little mistakes now and then is a given. That your fingers will get blisters and your brain will turn to mush is an occupational hazard.

But what makes it all worthwhile happens when you start to use the video display to show the results of your programs. The text-only video display we built in the last chapter has 1 kilobyte of random access memory that's used to store the ASCII codes of 25 lines of 40 characters each. A program writes to this memory the same way that it writes to any other memory in the computer.

But getting program output to the video display isn't as simple as it might first seem. If, for example, a program that you write does a particular calculation that results in the value 4Bh, you can't simply write that value to the video display memory. What you'll see in the screen in that case is the letter K because that's the letter that corresponds to the ASCII code 4Bh. Instead, you need to write two ASCII characters to the display: 34h, which is the ASCII code for 4, and 42h, which is the ASCII code for B. Each nibble of the 8-bit result is a hexadecimal digit, which must be displayed by the ASCII code for that digit.

Of course, you'll probably write little subroutines that perform this conversion. Here's one in 8080 assembly language that converts a nibble in the accumulator (assumed to be a value between 00h and 0Fh inclusive) to its ASCII equivalent:

NibbleToAscii: CPI A,0Ah ; Check if it's a letter or number

JC Number

ADD A,37h ; A to F converted to 41h to 46h

RET

Number: ADD A,30h ; 0 to 9 converted to 30h to 39h

RET

This subroutine