Code_ The Hidden Language of Computer Hardware and Software - Charles Petzold [148]
The MVI (Move Immediate) statement moves the value 9 into register C. The CALL 5 statement makes a CP/M function call. Function 9 means to display a string of characters beginning at the address given by the DE register pair and stop when a dollar sign is encountered. (You'll notice that the text in the last line of the program ends with a dollar sign. The use of a dollar sign to signify the end of a character string is quite odd, but that's the way CP/M happens to work.) The final RET statement ends the program and returns control to CP/M. (That's actually one of several ways to end a CP/M program.) The END statement indicates the end of the assembly-language file.
So we have a text file containing seven lines of text. The next step is to assemble it, which means to convert it to machine code. Previously we've done this by hand. But since we're running CP/M, we can use a program included with CP/M named ASM.COM. This is the CP/M assembler. We run ASM.COM from the CP/M command line this way:
ASM PROGRAM1.ASM
The ASM program looks at the file PROGRAM1.ASM and creates a new file named PROGRAM1.COM that contains the machine code corresponding to the assembly-language statements that we wrote. (Actually there's another step in the process, but it's not important in this account of what happens.) Now you can run PROGRAM1.COM from the CP/M command line. It displays the text "Hello!" and then ends.
The PROGRAM1.COM file contains the following 16 bytes:
11 09 01 OE 09 CD 05 00 C9 48 65 6C 6C 6F 21 24
The first 3 bytes are the LXI instruction, the next 2 the MVI instruction, the next 3 the CALL instruction, and the next the RET instruction. The last 7 bytes are the ASCII characters for the five letters of "Hello," the exclamation point, and the dollar sign.
What an assembler such as ASM.COM does is read an assembly-language program (often called a source-code file) and write out a file containing machine code—an executable file. In the grand scheme of things, assemblers are fairly simple programs because there's a one-to-one correspondence between the assembly-language mnemonics and machine code. The assembler works by separating each line of text into mnemonics and arguments and then comparing these small words and letters with a list that the assembler contains of all the possible mnemonics and arguments. These comparisons reveal which machine-code instructions correspond to each statement.
Notice how the assembler figures out that the LXI instruction must set the register pair DE to the address 0109h. If the LXI instruction itself is located at 0100h (as it is when CP/M loads the program into memory to run), address 0109h is where the text string begins. Generally a programmer using an assembler doesn't need to worry about the specific addresses associated with different parts of the program.
The first person to write the first assembler had to hand-assemble the program, of course. A person who writes a new (perhaps improved) assembler for the same computer can write it in assembly language and then use the first assembler to assemble it. Once the new assembler is assembled, it can assemble itself.
Every time a new microprocessor comes out, a new assembler is needed. The new assembler, however, can first be written on an existing computer using that computer's assembler. This is called a cross-assembler. The assembler runs on Computer A but creates code that runs on Computer B.
Although an assembler eliminates the less-creative aspects of assembly-language programming (the hand-assembling part), assembly language still has two major problems. The first problem (which you've perhaps already surmised) is that it can be very tedious. You're working down on the level of the microprocessor chip, and