SixtyPical/doc/SixtyPical.md

SixtyPical

This document describes the SixtyPical programming language version 0.6-PRE, both its execution aspect and its static analysis aspect (even though these are, technically speaking, separate concepts.)

This document is nominally normative, but the tests in the tests directory are even more normative.

Refer to the bottom of this document for an EBNF grammar of the syntax of the language.

Types

There are five TYPES in SixtyPical:

• bit (2 possible values)
• byte (256 possible values)
• byte table (256 entries, each holding a byte)
• routine (code stored somewhere in memory, read-only)
• vector (address of a routine)

Memory locations

A primary concept in SixtyPical is the MEMORY LOCATION. At any given point in time during execution, each memory location is either UNINITIALIZED or INITIALIZED. At any given point in the program text, too, each memory location is either uninitialized or initialized. Where-ever it is one or the other during execution, it is the same in the corresponding place in the program text; thus, it is a static property.

There are four general kinds of memory location. The first three are pre-defined and built-in.

Registers

Each of these hold a byte. They are initially uninitialized.

a
x
y

Flags

Each of these hold a bit. They are initially uninitialized.

c (carry)
z (zero)
v (overflow)
n (negative)

Constants

It may be strange to think of constants as memory locations, but keep in mind that a memory location in SixtyPical need not map to a memory location in the underlying hardware. All constants are read-only. Each is initially initialized with the value that corresponds with its name.

They come in bit and byte types. There are two bit constants,

off
on

and two-hundred and fifty-six byte constants,

0
1
...
255

User-defined

There may be any number of user-defined memory locations. They are defined by giving the type, which must be byte, byte table, or vector, and the name.

byte pos

A location in memory may be given explicitly on a user-defined memory location.

byte table screen @ 1024

Routines

Every routine must list all the memory locations it READS from, i.e. its INPUTS, and all the memory locations it WRITES to, whether they are OUTPUTS or merely TRASHED. Every memory location that is not written to by the routine (or any routines that the routine calls) is PRESERVED by the routine.

routine foo
  inputs a, score
  outputs x
  trashes y {
    ...
}

Routines may call only routines previously defined in the program source. Thus, recursive routines are not allowed.

For a SixtyPical program to be run, there must be one routine called main. This routine is executed when the program is run.

The memory locations given given as inputs are considered to be initialized at the beginning of the routine. Various instructions cause memory locations to be initialized after they are executed. Calling a routine which trashes some memory locations causes those memory locations to be uninitialized after that routine is called. At the end of a routine, all memory locations listed as outputs must be initialised.

A routine can also be declared as "external", in which case its body need not be defined but an absolute address must be given for where the routine is located in memory.

routine chrout
  inputs a
  trashes a
  @ 65490

Instructions

ld

ld <dest-memory-location>, <src-memory-location> [+ <index-memory-location>]

Reads from src and writes to dest.

• It is illegal if dest is not a register.
• It is illegal if dest does not occur in the WRITES lists of the current routine.
• It is illegal if src is not of same type as dest (i.e., is not a byte.)
• It is illegal if src is uninitialized.

After execution, dest is considered initialized. The flags z and n may be changed by this instruction; they must be named in the WRITES lists, and they are considered initialized after it has executed.

If and only if src is a byte table, the index-memory-location must be given.

Some combinations, such as ld x, y, are illegal because they do not map to underlying opcodes.

st

st <src-memory-location>, <dest-memory-location> [+ <index-memory-location>]

Reads from src and writes to dest.

• It is illegal if dest is a register or if dest is read-only.
• It is illegal if dest does not occur in the WRITES lists of the current routine.
• It is illegal if src is not of same type as dest.
• It is illegal if src is uninitialized.

After execution, dest is considered initialized. No flags are changed by this instruction (unless of course dest is a flag.)

If and only if dest is a byte table, the index-memory-location must be given.

add dest, src

add <dest-memory-location>, <src-memory-location>

Adds the contents of src to dest and stores the result in dest.

• It is illegal if src OR dest OR c is uninitialized.
• It is illegal if dest is read-only.
• It is illegal if dest does not occur in the WRITES lists of the current routine.

Affects n, z, c, and v flags, requiring that they be in the WRITES lists, and initializing them afterwards.

dest and src continue to be initialized afterwards.

inc

inc <dest-memory-location>

Increments the value in dest. Does not honour carry.

• It is illegal if dest is uninitialized.
• It is illegal if dest is read-only.
• It is illegal if dest does not occur in the WRITES lists of the current routine.

Affects n and z flags, requiring that they be in the WRITES lists, and initializing them afterwards.

sub

sub <dest-memory-location>, <src-memory-location>

Subtracts the contents of src from dest and stores the result in dest.

• It is illegal if src OR dest OR c is uninitialized.
• It is illegal if dest is read-only.
• It is illegal if dest does not occur in the WRITES lists of the current routine.

Affects n, z, c, and v flags, requiring that they be in the WRITES lists, and initializing them afterwards.

dest and src continue to be initialized afterwards.

dec

dec <dest-memory-location>

Decrements the value in dest. Does not honour carry.

• It is illegal if dest is uninitialized.
• It is illegal if dest is read-only.
• It is illegal if dest does not occur in the WRITES lists of the current routine.

Affects n and z flags, requiring that they be in the WRITES lists, and initializing them afterwards.

cmp

cmp <dest-memory-location>, <src-memory-location>

Subtracts the contents of src from dest (without considering carry) but does not store the result anywhere, only sets the resulting flags.

• It is illegal if src OR dest is uninitialized.

Affects n, z, and c flags, requiring that they be in the WRITES lists, and initializing them afterwards.

and, or, xor

and <dest-memory-location>, <src-memory-location>
or <dest-memory-location>, <src-memory-location>
xor <dest-memory-location>, <src-memory-location>

Applies the given bitwise Boolean operation to src and dest and stores the result in dest.

• It is illegal if src OR dest OR is uninitialized.
• It is illegal if dest is read-only.
• It is illegal if dest does not occur in the WRITES lists of the current routine.

Affects n and z flags, requiring that they be in the WRITES lists of the current routine, and sets them as initialized afterwards.

dest and src continue to be initialized afterwards.

shl, shr

shl <dest-memory-location>
shr <dest-memory-location>

shl shifts the dest left one bit position. The rightmost position becomes c, and c becomes the bit that was shifted off the left.

shr shifts the dest right one bit position. The leftmost position becomes c, and c becomes the bit that was shifted off the right.

• It is illegal if dest is a register besides a.
• It is illegal if dest is read-only.
• It is illegal if dest OR c is uninitialized.
• It is illegal if dest does not occur in the WRITES lists of the current routine.

Affects the c flag, requiring that it be in the WRITES lists of the current routine, and it continues to be initialized afterwards.

call

call <routine-name>

Just before the call,

• It is illegal if any of the memory locations in the called routine's READS list is uninitialized.

Just after the call,

• All memory locations listed as TRASHED in the called routine's WRITES list are considered uninitialized.
• All memory locations listed as TRASHED in the called routine's OUTPUTS list are considered initialized.

if

if <src-memory-location> {
    <true-branch>
} else {
    <false-branch>
}

Executes the true-branch if the value in src is nonzero, otherwise executes the false-branch. The false-branch is optional may be omitted; in this case it is treated like an empty block.

• It is illegal if src is not z, c, n, or v.
• It is illegal if src is not initialized.
• It is illegal if any location initialized at the end of the true-branch is not initialized at the end of the false-branch, and vice versa.

repeat

repeat {
    <block>
} until <src-memory-location>

Executes the block repeatedly until the src (observed at the end of the execution of the block) is non-zero. The block is always executed as least once.

• It is illegal if any memory location is uninitialized at the exit of the loop when that memory location is initialized at the start of the loop.

To simulate a "while" loop, use an if internal to the block, like

repeat {
    cmp y, 25
    if z {
    }
} until z

"until" is optional, but if omitted, must be replaced with "forever".

copy

copy <src-memory-location>, <dest-memory-location>

Reads from src and writes to dest. Differs from st in that is able to copy more general types of data (for example, vectors,) and it trashes the z and n flags and the a register.

• It is illegal if dest is read-only.
• It is illegal if dest does not occur in the WRITES lists of the current routine.
• It is illegal if src is not of same type as dest.
• It is illegal if src is uninitialized.

After execution, dest is considered initialized, and z and n, and a are considered uninitialized.

Grammar

Program ::= {Defn} {Routine}.
Defn    ::= "byte" ["table"] NewIdent ["@" WordConst].
Routine ::= "routine" NewIdent
            ["inputs" LocExprs] ["outputs" LocExprs] ["trashes" LocExprs]
            (Block | "@" WordConst).
LocExprs::= LocExpr {"," LocExpr}.
LocExpr ::= Register | Flag | LitByte | DefnIdent.
Register::= "a" | "x" | "y".
Flag    ::= "c" | "z" | "n" | "v".
LitByte ::= "0" ... "255".
LitWord ::= "0" ... "65535".
Block   ::= "{" {Instr} "}".
Instr   ::= "ld" LocExpr "," LocExpr ["+" LocExpr]
          | "st" LocExpr "," LocExpr ["+" LocExpr]
          | "add" LocExpr "," LocExpr
          | "sub" LocExpr "," LocExpr
          | "cmp" LocExpr "," LocExpr
          | "and" LocExpr "," LocExpr
          | "or" LocExpr "," LocExpr
          | "xor" LocExpr "," LocExpr
          | "shl" LocExpr
          | "shr" LocExpr
          | "inc" LocExpr
          | "dec" LocExpr
          | "call" RoutineIdent
          | "if" ["not"] LocExpr Block ["else" Block]
          | "repeat" Block ("until" ["not"] LocExpr | "forever")
          | "copy" LocExpr "," LocExpr ["+" LocExpr]
          .