7.36

Special Section: Building Your Own Computer 323 Machine-Language Programming) Let's create a computer called the Simpletron. As its lies, it's a simple machine, but powerful. The Simpletron runs programs written in the name im ls l anguage it directly understands: Simpletron Machine Language (SML.). impletron contains an accumulator-a special register in which information is pu letron uses that information in calculations or examines it in various ways tion in the Simpletron is handled in terms of words. A word is a signed four-digit decim before the Simpletron number, such as +3364, -1293, -0007 and -0001. The Simpletron is equipped with a 100-word and these words are referenced by their location numbers 00, 01,... 99. running an SML program, we must load, or place, the program into memory. The first instruction (or statement) of every SML program is always placed in location 00. The simulator will start executing at this location. Each instruction written in SML occupies one word of the Simpletron's memory (so instructions are signed four-digit decimal numbers). We shall assume that the sign of an SML instruction is always plus, but the sign of a data word may be either plus or minus. Each location in the Simpletron's mem- ory may contain an instruction, a data value used by a program or an unused (and so undefined) area of memory. The first two digits of each SML instruction are the operation code specifying the opera- tion to be performed. SML operation codes are summarized in Fig. 7.33. The last two digits of an SML instruction are the operand the address of the memory location containing the word to which the operation applies. Let's consider several simple SML programs. Operation code Meaning Inputloutput operations