Target Machine Description for 488/2107

Spring 1998

1. A main memory, consisting of memorySize words, numbered 0 to memorySize - 1 . A number specifying a word in memory is called an address. Each word is 16 bits. The bit-pattern in a word may be interpreted in any of the following ways:
   1. An integer value in the range -32767 to +32767. The representation is two's complement.
   2. A boolean value. False is represented the same as integer 0; true is represented the same as integer 1.
   3. A character value (one character per word). The representation is ASCII, extended to 16 bits by prefixing 8 zeros.
   4. The undefined data value represented by -32768 .
   5. An operation code. The operations and codes are listed later.
   6. A memory address.
2. A 16-bit register pc (program counter), which contains the address of the next instruction to be executed.
3. A 16-bit register mt (memory top), which contains the address of the next free word of memory.
4. Some 16-bit registers display, numbered 0 to displaySize - 1 . These are used to make the variables of visible lexical levels addressable.

At the start of execution, the following may be presumed.

1. The instructions of a program occupy contiguous memory locations starting at word 0.
2. Register mt contains the address of the first word after the instructions.
3. The rest of memory contains garbage.
4. The display contains garbage.
5. pc contains the address of the first instruction to be executed.

Beginning after the instructions, space is allocated for variables, and made addressable via the display , as required. Space required for intermediate results in the evaluation of expressions is at the top of the used portion of memory (the Evaluation Stack).

An instruction may occupy one, two, or three words of memory. The first word contains an op-code, specifying which machine operation to perform. The second and third words, if present, contain operands. In the following description, push, pop, and top refer to the memory considered as a stack whose top pointer is mt. Variables a, v, n, x and y are local to the description.

Machine Instructions

0	ADDR LL ON    push( display[ LL ]+ON )
1	LOAD	      a:= top; pop;
		      if memory[ a ]=undefined then error;
		      push( memory[ a ] )
2	STORE	      v:= top; pop;
		      a:= top; pop;
		      memory[ a ]:= v
3	PUSH V	      push( V )
4	PUSHMT	      push( mt )
5	SETD LL	      display[ LL ]:= top; pop
6	POPN	      n:= top; pop; pop n times
7	POP	      pop
8	DUPN	      n:= top; pop; v:= top; pop; push( v ) n times
9	DUP	      push( top )
10	BR 	      a:= top; pop; go to a
11	BF	      a:= top; pop; v:= top; pop; if Bv then go to a
12	NEG	      top := \(mi top 
13	ADD	      For operations 13-19:
14	SUB		   y:= top; pop;
15	MUL		   x:= top; pop;
16	DIV		   n := x op y ;
17	EQ		   if overflow or zerodivide then error else push( n ) ;
18	LT		
19	OR	
20	SWAP	      x:= top; pop; y:= top; pop; push( x ); push(y)
21	READC	      One character of input is read and pushed.
22	PRINTC	      Print top as character; pop
23	READI	      Starting at the current character of input, blanks and line boundaries are read and
		      ignored up to the next digit or sign ( if first non-blank is something else then error );
		      Then consecutive digits are read up to the first non-digit;  They are converted to an
		      integer n;  push( n ).
24	PRINTI	      Print top as integer; pop
25	HALT	      Halt

Note:	The following two pseudo instructions, TRON and TROFF,  are provided as aids to 
	assist in the debugging of the code generator.


26	TRON          Start tracing the execution of machine instructions if tracing has been enabled.
27	TROFF	      Stop tracing the execution of machine instructions.  Non-op if tracing not enabled.