Programmer 7500

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Text File | 1987-12-23 | 13.2 KB | 353 lines

The first 8 bytes in the system are vectors to the Cold and Warm start entries. You can freely jump to them in code anytime. The DPUSH and HPUSH labels are space savers. We jump to them in several CODE words when we want to push their contents on the Parameter Stack. >NEXT is where all the action is. It is the guts of the Forth Virtual Machine. It must advance the interpretive pointer held in the IP register pair and jump indirect to what it points to. We define a few macros here to make our life a little easier later. Using NEXT as a macro allows us to put it inline later. RP Used to hold the depth of the return stack NEST The runtime code for : It pushs the current IP onto the return stack and sets the IP to point to the parameter field of the word being executed. EXIT Pop an entry off the return stack and place it into the Interpretive Pointer. Terminates a Hi Level definition. UNNEST Same as exit. Compiled by ; to help decompiling. DODOES The runtime portion of defining words. First it pushes the IP onto the return stack and then it pushes the BODY address of the word being executed onto the parameter stack. DOCREATE Leave a pointer to its own parameter field on the stack. This is also the runtime for variable. UP Holds a pointer to the current USER area. ( multitasking ) @USER A subroutine called from code level words that returns the contents of a particular user variable. !USER A subroutine called from code level words that sets the contents of a particular user variable. DOCONSTANT The run time code for CONSTANT. It takes the contents of the parameter field and pushes it onto the stack. DOUSER The run time code for USER variables. Places a pointer to the current version of this variable on the stack. Needed for multitasking. (LIT) The runtime code for literals. Pushes the following two bytes onto the parameter stack and moves the IP over them. It is compiled by the word LITERAL. BRANCH Performs an unconditional branch. Notice that we are using absolute addresses insead of relative ones. (fast) ?BRANCH Performs a conditional branch. If the top of the parameter stack in True, take the branch. If not, skip over the branch address which is inline. LOOP-EXIT is a common routine used by (LOOP) and (+LOOP) It is called when the loop has terminated and is exited normally. (LOOP) the runtime procedure for LOOP. Branches back to the beginning of the loop if there are more iterations to do. Otherwise it exits. The loop counter is incremented. LOOP-BRANCH A common routine needed twice in the 8080 implementation of (+LOOP). (+LOOP) Increment the loop counter by the value on the stack and decide whether or not to loop again. Due to the wierdness of the 8080, you have to stand on your head to determine the conditions under which you loop or exit. (DO) The runtime code compiled by DO. Pushes the inline address onto the return stack along with values needed by (LOOP). (?DO) The runtime code compiled by ?DO. The difference between ?DO and DO is that ?DO will not perform any iterations if the initial index is equal to the final index. BOUNDS Given address and length, make it ok for DO ... LOOP. These are again the TRANSITION versions of the immediate words for looping. They compile the correct run time code and then Mark and Resolve the various branches. >NEXT The address of the inner interpreter. EXECUTE the word whose code field is on the stack. Very useful for passing executable routines to procedures!!! PERFORM the word whose code field is stored at the address pointed to by the number on the stack. Same as @ EXECUTE DO-DEFER The runtime code for deferred words. Fetches the code field and executes it. DOUSER-DEFER The runtime code for User deferred words. These are identical to regular deferred words except that each task has its own version. GO Execute code at the given address. NOOP One of the most useful words in Forth. Does nothing. PAUSE Used by the Multitasker to switch tasks. I returns the current loop index. It now requires a little more calculation to compute it than in FIG Forth but the tradeoff is a much faster (LOOP). The loop index is stored on the Return Stack. J returns the loop index of the inner loop in nested DO .. LOOPs. (LEAVE) Does an immediate exit of a DO ... LOOP structure. Unlike FIG Forth which waits until the next LOOP is executed. (?LEAVE) Leaves if the flag on the stack is true. Continues if not. LEAVE I have to do this to be 83-Standard. @ Fetch a 16 bit value from addr. ! Store a 16 bit value at addr. C@ Fetch an 8 bit value from addr. C! Store an 8 bit value at addr. CMOVE Move a set of bytes from the from address to the to address. The number of bytes to be moved is count. The bytes are moved from low address to high address, so overlap is possible and in fact sometimes desired. CMOVE> The same as CMOVE above except that bytes are moved in the opposite direction, ie from high addresses to low addresses. SP@ Return the address of the next entry on the parameter stack SP! ( Warning, this is different from FIG Forth ) Sets the parameter stack pointer to the specified value. RP@ Return the address of the next entry on the return stack. RP! ( Warning, this is different from FIG Forth ) Sets the return stack pointer to the specified value. DROP Throw away the top element of the stack. DUP Duplicate the top element of the stack. SWAP Exchange the top two elements on the stack. OVER Copy the second element to the top. TUCK Tuck the first element under the second one. NIP Drop the second element from the stack. ROT Rotate the top three element, bringing the third to the top. -ROT The inverse of ROT. Rotates the top element to third place. FLIP Exhange the hi and low halves of a word. ?DUP Duplicate the top of the stack if it is non-zero. R> Pops a value off of the return stack and pushes it onto the parameter stack. It is dangerous to use this randomly! >R Pops a value off of the parameter stack and pushes it onto return stack. It is dangerous to use this randomly! R@ Copies the value on the return stack to the parameter stack. PICK Reaches into the stack and grabs an element, copying it to the top of the stack. For example, if the stack has 1 2 3 Then 0 PICK is 3, 1 PICK is 2, and 2 PICK is 1. ROLL Similar to SHAKE and RATTLE. Should be avoided. 1 ROLL is SWAP, 2 ROLL is ROT, etc. ROLL can be useful, but it is slow. AND Returns the bitwise AND of n1 and n2 on the stack. OR Returns the bitwise OR of n1 and n2 on the stack. XOR Returns the bitwise Exclusive Or of n1 and n2 on the stack. NOT Does a ones complement of the top. Equivalent to -1 XOR. TRUE FALSE Constants for clarity. YES Push a true flag on the stack and jump to next NO Push a false flag on the stack and jump to next CSET Set the contents of addr so that the bits that are 1 in n are also 1 in addr. Equivalent to DUP C@ ROT OR SWAP C! CRESET Set the contents of addr so the the bits that are 1 in n are zero in addr. Equivalent to DUP C@ ROT NOT AND SWAP C! CTOGGLE Flip the bits in addr by the value n. Equivalent to DUP C@ ROT XOR SWAP C! ON Set the contents of addr to TRUE OFF Set the contents of addr to FALSE + Add the top two numbers on the stack and return the result. NEGATE Turn the number into its negative. A twos complement op. - Subtracts n2 from n1 leaving the result on the stack. ABS Return the absolute value of the 16 bit integer on the stack +! Increment the value at addr by n. This is equivalent to the following: DUP @ ROT + SWAP ! but much faster. 0 1 Frequently used constants 2 3 Are faster and more code efficient. 2* Double the number on the Stack. 2/ Shift the number on the stack right one bit. Equivalent to division by 2 for positive numbers. U2/ 16 bit logical right shift. 8* Multiply the top of the stack by 8. 1+ Increment the top of the stack by one. 2+ Increment the top of the stack by two. 1- Decrement the top of the stack by one. 2- Decrement the top of the stack by two. You could write a whole book about multiplication and division, and in fact Knuth did. Suffice it to say that UM* is the basic multiplication primitive in Forth. It takes two unsigned 16 bit integers and returns an unsigned 32 bit result. All other multiplication functions are derived from this primitive one. It probably isn't particularly fast or elegant, but that is because I never liked arithmetic and I stole this implementation from FIG Forth anyway. U*D is a synonym for UM* These are various subroutines used by the division primitive in Forth, namely U/. Again I must give credit for them to FIG Forth, since if I can't even understand multiply, divide would be completely hopeless. UM/MOD This is the division primitive in Forth. All other division operations are derived from it. It takes a double number, d1, and divides by by a single number n1. It leaves a remainder and a quotient on the stack. For a clearer understanding of arithmetic consult Knuth Volume 2 on Seminumerical Algorithms. 0= Returns True if top is zero, False otherwise. 0< Returns true if top is negative, ie sign bit is on. 0> Returns true if top is positive. 0<> Returns true if the top is non-zero, False otherwise. = Returns true if the two elements on the stack are equal, False otherwise. <> Returns true if the two element are not equal, else false. ?NEGATE Negate the second element if the top is negative. U< Compare the top two elements on the stack as unsigned integers and return true if the second is less than the first. Be sure to use U< whenever comparing addresses, or else strange things will happen beyond 32K. U> Compare the top two elements on the stack as unsigned integers. True if n1 > n2 unsigned. < Compare the top two elements on the stack as signed integers and return true if n1 < n2. > Compare the top two elements on the stack as signed integers and return true if n1 > n2. MIN Return the minimum of n1 and n2 MAX Return the maximum of n1 and n2 BETWEEN Return true if min <= n1 <= max, otherwise false. WITHIN Return true if min <= n1 < max, otherwise false. 2@ Fetch a 32 bit value from addr. 2! Store a 32 bit value at addr. 2DROP Drop the top two elements of the stack. 2DUP Duplicate the top two elements of the stack. 2SWAP Swap the top two pairs of numbers on the stack. You can use this operator to swap two 32 bit integers and preserve their meaning as double numbers. 2OVER Copy the second pair of numbers over the top pair. Behaves like 2SWAP for 32 bit integers. 3DUP Duplicate the top three elements of the stack. 4DUP Duplicate the top four elements of the stack. 2ROT rotates top three double numbers. D+ Add the two double precision numbers on the stack and return the result as a double precision number. DNEGATE Same as NEGATE except for double precision numbers. S>D Take a single precision number and make it double precision by extending the sign bit to the upper half. DABS Return the absolute value of the 32 bit integer on the stack D2* 32 bit left shift. D2/ 32 bit arithmetic right shift. Equivalent to divide by 2. D- Subtract the two double precision numbers. ?DNEGATE Negate the double number if the top is negative. D0= Compare the top double number to zero. True if d = 0 D= Compare the top two double numbers. True if d1 = d2 DU< Performs unsigned comparison of two double numbers. D< Compare the top two double numbers. True if d1 < d2 D> Compare the top two double numbers. True if d1 > d2 DMIN Return the lesser of the top two double numbers. DMAX Return the greater of the the top two double numbers. This does all the arithmetic you could possibly want and even more. I can never remember exactly what the order of the arguments is for any of these, except maybe * / and MOD, so I suggest you just try it when you are in doubt. That is one of the nice things about having an interpreter around, you can ask it questions anytime and it will tell you the answer. *D multiplys two singles and leaves a double. M/MOD divides a double by a single, leaving a single quotient and a single remainder. Division is floored. MU/MOD divides a double by a single, leaving a double quotient and a single remainder. Division is floored. */ is a particularly useful operator, as it allows you to do accurate arithmetic on fractional quantities. Think of it as multiplying n1 by the fraction n2/n3. The intermediate result is kept to full accuracy. Notice that this is not the same as * followed by /. See Starting Forth for more examples.