VTL02 (VTL-2 for the 6502) by Mike Barry
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;234567890123456789012345678901234567890123456789012345
.lf vtl02ba2.lst
.cr 6502
.tf vtl02ba2.obj,ap1
;-----------------------------------------------------;
; VTL-2 for the 6502 (VTL02B) ;
; Original Altair 680b version by ;
; Frank McCoy and Gary Shannon 1977 ;
; 2012: Adapted to the 6502 by Michael T. Barry ;
; Thanks to sbprojects.com for a very nice assembler! ;
;-----------------------------------------------------;
; 2015: Revision B, with several space optimizations
; (suggested by dclxvi) and enhancements (suggested
; by mkl0815 and Klaus2m5).
;
; New features in Revision B:
; * Bit-wise operators & | ^ (and, or, xor)
; Example: A=$|128) Get a char and set hi-bit
;
; * Absolute addressed 8-bit memory load and store
; via the {< @} facility:
; Example: <=P) Point to the I/O port at P
; @=@&254^128) Clear low-bit & flip hi-bit
;
; * The space character is no longer a valid user
; variable nor a "valid" binary operator. It is
; now only significant as a numeric constant
; terminator, and as a place-holder in strings and
; program listings, where it may be used to improve
; human readability, at a slight cost in execution
; speed and memory consumption.
; Example:
; * (VTL-2)
; 1000 A=1) Init loop index
; 1010 ?=A) Print index
; 1020 ?="") Newline
; 1030 A=A+1) Update index
; 1040 #=A<10*1010) Loop until done
;
; * (VTL02B)
; 1000 A = 1 ) Init loop index
; 1010 ? = A ) Print index
; 1020 ? = "" ) Newline
; 1030 A = A + 1 ) Update index
; 1040 # = A < 10 * 1010 ) Loop until done
;-----------------------------------------------------;
; Copyright (c) 2012, Michael T. Barry
; Revision B (c) 2015, Michael T. Barry
; All rights reserved.
;
; Redistribution and use in source and binary forms,
; with or without modification, are permitted,
; provided that the following conditions are met:
;
; 1. Redistributions of source code must retain the
; above copyright notice, this list of conditions
; and the following disclaimer.
; 2. Redistributions in binary form must reproduce the
; above copyright notice, this list of conditions
; and the following disclaimer in the documentation
; and/or other materials provided with the
; distribution.
;
; THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS
; AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED
; WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
; IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
; FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT
; SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE
; FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
; EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
; NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
; SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
; INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
; LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
; OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
; IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
; ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
;
; Notes concerning this version:
; * {&} and {*} are initialized on entry.
; * Division by zero returns a quotient of 65535
; (the original 6800 version froze).
; * The 6502 has NO 16-bit registers (other than PC)
; and less overall register space than the 6800,
; so the interpreter reserves some obscure VTL02B
; variables {@ $ ( ) 0 1 2 3 4 5 6 7 8 9 < > : ?}
; for its internal use (the 680b version used a
; similar tactic, but differed in the details).
; The deep nesting of parentheses also puts {; < =}
; in danger of corruption. For example, executing
; the statement A=((((((((1)))))))) sets both {A}
; and {;} to the value 1.
; * Users wishing to call a machine language subroutine
; via the system variable {>} must first set the
; system variable {"} to the proper address vector
; (for example, "=768).
; * The x register is used to point to a simple VTL02B
; variable (it can't point explicitly to an array
; element like the 680b version because it's only
; 8-bits). In the comments, var[x] refers to the
; 16-bit contents of the zero-page variable pointed
; to by register x (residing at addresses x, x+1).
; * The y register is used as a pointer offset inside
; a VTL02B statement (easily handling the maximum
; statement length of about 128 bytes). In the
; comments, @[y] refers to the 16-bit address
; formed by adding register y to the value in {@}.
; * The structure and flow of this interpreter is
; similar to the 680b version, but it has been
; reorganized in a more 6502-friendly format (the
; 6502 has no 'bsr' instruction, so the 'stuffing'
; of subroutines within 128 bytes of the caller is
; only advantageous for conditional branches).
; * I designed this version to duplicate the OFFICIALLY
; DOCUMENTED behavior of Frank's 680b version:
; http://www.altair680kit.com/manuals/Altair_
; 680-VTL-2%20Manual-05-Beta_1-Searchable.pdf
; Both versions ignore all syntax errors and plow
; through VTL-2 programs with the assumption that
; they are "correct", but in their own unique ways,
; so any claims of compatibility are null and void
; for VTL-2 code brave (or stupid) enough to stray
; from the beaten path.
; * This version is wound rather tightly, in a failed
; attempt to fit it into 768 bytes like the 680b
; version; many structured programming principles
; were sacrificed in that effort. The 6502 simply
; requires more instructions than the 6800 does to
; manipulate 16-bit quantities, but the overall
; execution speed should be comparable due to the
; 6502's slightly lower average clocks/instruction
; ratio. As it is now, it fits into 1KB with room
; to spare. I coded to emphasize compactness over
; execution speed at every perceived opportunity,
; but may have missed some optimizations. Further
; suggestions are welcome.
; * VTL02B is my free gift (?) to the world. It may be
; freely copied, shared, and/or modified by anyone
; interested in doing so, with only the stipulation
; that any liabilities arising from its use are
; limited to the price of VTL02B (nothing).
;-----------------------------------------------------;
; VTL02B variables occupy RAM addresses $0080 to $00ff,
; and are little-endian, in the 6502 tradition.
; The use of lower-case and some control characters for
; variable names is allowed, but not recommended; any
; attempts to do so would likely result in chaos, due
; to aliasing with upper-case and system variables.
; Variables tagged with an asterisk are used internally
; by the interpreter and may change without warning.
; {@ $ ( ) 0..9 : > ?} are (usually) intercepted by
; the interpreter, so their internal use by VTL02B is
; "safe". The same cannot be said for {; < =}, so be
; careful!
at = $80 ; {@}* internal pointer / mem byte
; VTL02B standard user variable space
; {A B C .. X Y Z [ \ ] ^ _}
; VTL02B system variable space
space = $c0 ; { } New for VTL02B: the space
; character is no longer a valid
; user variable nor a "valid"
; binary operator. It is now
; only significant as a numeric
; constant terminator, and as a
; place-holder in strings and
; program listings.
bang = $c2 ; {!} return line number
quote = $c4 ; {"} user ml subroutine vector
pound = $c6 ; {#} current line number
dolr = $c8 ; {$}* temp storage / char i/o
remn = $ca ; {%} remainder of last division
ampr = $cc ; {&} pointer to start of array
tick = $ce ; {'} pseudo-random number
lparen = $d0 ; {(}* old line # / begin sub-exp
rparen = $d2 ; {)}* temp storage / end sub-exp
star = $d4 ; {*} pointer to end of free mem
; $d6 ; {+ , - . /} valid variables
; Interpreter argument stack space
arg = $e0 ; {0 1 2 3 4 5 6 7 8 9 :}*
; Rarely used variables and argument stack overflow
; $f6 ; {;}* valid user variable
lthan = $f8 ; {<}* user memory byte pointer
; = $fa ; {=}* valid user variable
gthan = $fc ; {>}* temp / call ML subroutine
ques = $fe ; {?}* temp / terminal i/o
;
nulstk = $01ff ; system stack resides in page 1
;-----------------------------------------------------;
; Equates for a 48K+ Apple 2 (original, +, e, c, gs)
ESC = 27 ; "Cancel current input line" key
BS = 8 ; "Delete last keypress" key
OP_OR = '!' ; Bit-wise OR operator
linbuf = $0200 ; input line buffer
prgm = $0800 ; VTL02B program grows from here
himem = $8000 ; ... up to the top of user RAM
vtl02b = $8000 ; interpreter cold entry point
; (warm entry point is startok)
KBD = $c000 ; 128 + keypress if waiting
KEYIN = $fd0c ; apple monitor keyin routine
COUT = $fded ; apple monitor charout routine
;=====================================================;
.or vtl02b
;-----------------------------------------------------;
; Initialize program area pointers and start VTL02B
; 17 bytes
lda #prgm
sta ampr ; {&} -> empty program
lda /prgm
sta ampr+1
lda #himem
sta star ; {*} -> top of user RAM
lda /himem
sta star+1
startok:
sec ; request "OK" message
; - - - - - - - - - - - - - - - - - - - - - - - - - - ;
; Start/restart VTL02B command line with program intact
; 29 bytes
start:
cld
ldx #nulstk
txs ; reset the system stack pointer
bcc user ; skip "OK" if carry clear
jsr outnl
lda #'O' ; output \nOK\n to terminal
jsr outch
lda #'K'
jsr outch
user:
jsr newln ; input a line from the user
ldx #pound ; cvbin destination = {#}
jsr cvbin ; does line start with a number?
bne stmnt ; yes: handle program line
; no: execute direct statement
; - - - - - - - - - - - - - - - - - - - - - - - - - - ;
; The main program execution loop
; 49 bytes
eloop:
php ; (cc: deferred, cs: direct)
jsr exec ; execute one VTL02B statement
plp
lda pound ; (eq) if {#} = 0
ora pound+1
bcc eloop2 ; if direct mode and {#} = 0
beq start ; then restart cmd prompt
clc ; if direct mode and {#} <> 0
bne xloop ; then start execution @ {#}
eloop2:
sec ; if program mode and {#} = 0
beq xloop ; then execute next line
lda pound+1 ; (false branch condition)
cmp lparen+1
bne branch ; else has {#} changed?
lda pound
cmp lparen
beq xloop ; no: execute next line (cs)
branch:
ldy lparen+1
ldx lparen ; yes: execute a VTL02B branch
inx ; (cs: forward, cc: backward)
bne branch2 ; {!} = {(} + 1 (return ptr)
iny
branch2:
stx bang
sty bang+1
xloop:
jsr findln ; find first/next line >= {#}
iny ; point to left-side of statement
bne eloop ; execute statement at new {#}
;-----------------------------------------------------;
; Delete/insert/replace program line or list program
; 7 bytes
stmnt:
clc
lda pound ; {#} = 0?
ora pound+1 ; no: delete/insert/replace line
bne skp2 ; yes: list program to terminal
; - - - - - - - - - - - - - - - - - - - - - - - - - - ;
; List program to terminal and restart "OK" prompt
; entry: Carry must be clear
; uses: findln, outch, prnum, prstr, {@ ( )}
; 20 bytes
list_:
jsr findln ; find program line >= {#}
ldx #lparen ; line number for prnum
jsr prnum ; print the line number
lda #' ' ; print a space instead of the
jsr outch ; line length byte
lda #0 ; zero for delimiter
jsr prstr ; print the rest of the line
bcs list_ ; (always taken)
;-----------------------------------------------------;
; Delete/insert program line and restart command prompt
; entry: Carry must be clear
; uses: find, start, {@ > # & * (}, linbuf
; 155 bytes
skp2:
tya ; save linbuf offset pointer
pha
jsr find ; point {@} to first line >= {#}
bcs insrt
lda lparen
cmp pound ; if line doesn't already exist
bne insrt ; then skip deletion process
lda lparen+1
eor pound+1
bne insrt
tax ; x = 0
lda (at),y
tay ; y = length of line to delete
eor #-1
adc ampr ; {&} = {&} - y
sta ampr
bcs delt
dec ampr+1
delt:
lda at
sta gthan ; {>} = {@}
lda at+1
sta gthan+1
delt2:
lda gthan
cmp ampr ; delete the line
lda gthan+1
sbc ampr+1
bcs insrt
lda (gthan),y
sta (gthan,x)
inc gthan
bne delt2
inc gthan+1
bcc delt2 ; (always taken)
insrt:
pla
tax ; x = linbuf offset pointer
lda pound
pha ; push the new line number on
lda pound+1 ; the system stack
pha
ldy #2
cntln:
inx
iny ; determine new line length in y
lda linbuf-1,x ; and push statement string on
pha ; the system stack
bne cntln
cpy #4 ; if empty line then skip the
bcc jstart ; insertion process
tax ; x = 0
tya
clc
adc ampr ; calculate new program end
sta gthan ; {>} = {&} + y
txa
adc ampr+1
sta gthan+1
lda gthan
cmp star ; if {>} >= {*} then the program
lda gthan+1 ; won't fit in available RAM,
sbc star+1 ; so dump the stack and abort
bcs jstart ; to the "OK" prompt
slide:
lda ampr
bne slide2
dec ampr+1
slide2:
dec ampr
lda ampr
cmp at
lda ampr+1
sbc at+1
bcc move ; slide open a gap inside the
lda (ampr,x) ; program just big enough to
sta (ampr),y ; hold the new line
bcs slide ; (always taken)
move:
tya
tax ; x = new line length
move2:
pla ; pull the statement string and
dey ; the new line number and store
sta (at),y ; them in the program gap
bne move2
ldy #2
txa
sta (at),y ; store length after line number
lda gthan
sta ampr ; {&} = {>}
lda gthan+1
sta ampr+1
jstart:
jmp start ; dump stack, restart cmd prompt
;-----------------------------------------------------;
; Point @[y] to the first/next program line >= {#}
; entry: (cc): start search at beginning of program
; (cs): start search at next line
; ({@} -> beginning of current line)
; uses: find, jstart, prgm, {@ # & (}
; exit: if line not found then abort to "OK" prompt
; else {@} -> found line, {#} = {(} = actual
; line number, y = 2, (cc)
; 14 bytes
findln:
jsr find ; find first/next line >= {#}
bcs jstart ; if end then restart "OK" prompt
lda lparen
sta pound ; {#} = {(}
lda lparen+1
sta pound+1
rts
;-----------------------------------------------------;
; {?="...} handler; called from 'exec'
; List line handler; called from 'list_'
; 2 bytes
prstr:
iny ; skip over the " or length byte
tax ; x = delimiter, fall through
; - - - - - - - - - - - - - - - - - - - - - - - - - - ;
; Print a string at @[y]
; x holds the delimiter char, which is skipped over,
; not printed (a null byte is always a delimiter)
; If a key was pressed, it pauses for another keypress
; before returning. If either of those keys was a
; ctrl-C, it dumps the stack and restarts the "OK"
; prompt with the user program intact
; entry: @[y] -> string, x = delimiter char
; uses: inch, inkey, jstart, outch, execrts
; exit: (normal) @[y] -> null or byte after delimiter
; (ctrl-C) dump the stack & restart "OK" prompt
; 39 bytes
prmsg:
txa
cmp (at),y ; found delimiter or null?
beq prmsg2 ; yes: finish up
lda (at),y
beq prmsg2
jsr outch ; no: print char to terminal
iny ; and loop (with safety escape)
bpl prmsg
prmsg2:
tax ; save closing delimiter
jsr inkey ; any key = pause?
bcc prout ; no: proceed
jsr inch ; yes: wait for another key
prout:
txa ; retrieve closing delimiter
beq outnl ; always \n after null delimiter
jsr skpbyte ; skip over the delimiter
cmp #';' ; if trailing char is ';' then
beq execrts ; suppress the \n
outnl:
lda #$0d ; \n to terminal
joutch:
jmp outch
;-----------------------------------------------------;
; Execute a (hopefully) valid VTL02B statement at @[y]
; entry: @[y] -> left-side of statement
; uses: nearly everything
; exit: note to machine language subroutine {>=...}
; users: no registers or variables are
; required to be preserved except the system
; stack pointer, the text base pointer {@},
; and the original line number {(}
; if there is a {"} directly after the assignment
; operator, the statement will execute as {?="...},
; regardless of the variable named on the left side
; 90 bytes
exec:
jsr getbyte ; fetch left-side variable name
beq execrts ; do nothing if null statement
iny
ldx #arg ; initialize argument pointer
jsr convp ; arg[{0}] = address of left-side
bne exec1 ; variable
lda arg
cmp #rparen ; full line comment?
beq execrts ; yes: do nothing with the rest
exec1:
jsr getbyte ; skip over assignment operator
jsr skpbyte ; is right-side a literal string?
cmp #'"' ; yes: print the string with
beq prstr ; trailing ';' check & return
ldx #arg+2 ; point eval to arg[{1}]
jsr eval ; evaluate right-side in arg[{1}]
lda arg+2
ldx arg+1 ; was left-side an array element?
bne exec3 ; yes: skip to default actions
ldx arg
cpx #at ; if (@=...} statement then poke
bne exec1a ; low half of arg[{1}] to ({<})
ldy #0
sta (lthan),y
rts
exec1a:
cpx #dolr ; if {$=...} statement then print
beq joutch ; arg[{1}] as ascii character
cpx #gthan
bne exec2 ; if {>=...} statement then call
tax ; user machine language routine
lda arg+3 ; with arg[{1}] in a, x regs
jmp (quote) ; (MSB, LSB)
exec2:
cpx #ques ; if {?=...} statement then print
beq prnum0 ; arg[{1}] as unsigned decimal
exec3:
ldy #0
sta (arg),y
adc tick+1 ; store arg[{1}] in the left-side
rol ; variable
tax
iny
lda arg+3
sta (arg),y
adc tick ; pseudo-randomize {'}
rol
sta tick+1
stx tick
execrts:
rts
;-----------------------------------------------------;
; {?=...} handler; called by 'exec'
; 2 bytes
prnum0:
ldx #arg+2 ; x -> arg[{1}], fall through
; - - - - - - - - - - - - - - - - - - - - - - - - - - ;
; Print an unsigned decimal number (0..65535) in var[x]
; entry: var[x] = number to print
; uses: div, outch, var[x+2], preserves original {%}
; exit: var[x] = 0, var[x+2] = 10
; 43 bytes
prnum:
lda remn
pha ; save {%}
lda remn+1
pha
lda #10 ; divisor = 10
sta 2,x
lda #0
pha ; null delimiter for print
sta 3,x ; repeat {
prnum2:
jsr div ; divide var[x] by 10
lda remn
ora #'0' ; convert remainder to ascii
pha ; stack digits in ascending
lda 0,x ; order ('0' for zero)
ora 1,x
bne prnum2 ; } until var[x] is 0
pla
prnum3:
jsr outch ; print digits in descending
pla ; order until delimiter is
bne prnum3 ; encountered
pla
sta remn+1 ; restore {%}
pla
sta remn
rts
;-----------------------------------------------------;
; Evaluate a (hopefully) valid VTL02 expression at @[y]
; and place its calculated value in arg[x]
; A VTL02B expression is defined as a string of one or
; more terms, separated by operators and terminated
; with a null or an unmatched right parenthesis
; A term is defined as a variable name, a decimal
; constant, or a parenthesized sub-expression; terms
; are evaluated strictly from left to right
; A variable name is defined as a user variable, an
; array element expression enclosed in {: )}, or a
; system variable (which may have side-effects)
; entry: @[y] -> expression text, x -> argument
; uses: getval, oper, {@}, argument stack area
; exit: arg[x] = result, @[y] -> next text
; 31 bytes
eval:
lda #0
sta 0,x ; start evaluation by simulating
sta 1,x ; {0+expression}
lda #'+'
notdn:
pha ; stack alleged operator
inx ; advance the argument stack
inx ; pointer
jsr getval ; arg[x+2] = value of next term
dex
dex
pla ; retrieve and apply the operator
jsr oper ; to arg[x], arg[x+2]
jsr getbyte ; end of expression?
beq evalrts ; (null or right parenthesis)
iny
cmp #')' ; no: skip over the operator
bne notdn ; and continue the evaluation
evalrts:
rts ; yes: return with final result
;-----------------------------------------------------;
; Get numeric value of the term at @[y] into var[x]
; Some examples of valid terms: 123, $, H, (15-:J)/?)
; 83 bytes
getval:
jsr cvbin ; decimal number at @[y]?
bne getrts ; yes: return with it in var[x]
jsr getbyte
iny
cmp #'?' ; user line input?
bne getval2
tya ; yes:
pha
lda at ; save @[y]
pha ; (current expression ptr)
lda at+1
pha
jsr inln ; input expression from user
jsr eval ; evaluate, var[x] = result
pla
sta at+1
pla
sta at ; restore @[y]
pla
tay
rts ; skip over "?" and return
getval2:
cmp #'$' ; user char input?
bne getval2a
jsr inch ; yes: input one char
bcs getval5 ; (always taken)
getval2a:
cmp #'@' ; memory access?
bne getval3
sty dolr ; yes:
ldy #0
lda (lthan),y ; access memory byte at ({<})
ldy dolr
bne getval5 ; (always taken)
getval3:
cmp #'(' ; sub-expression?
beq eval ; yes: evaluate it recursively
jsr convp ; no: first set var[x] to the
lda (0,x) ; named variable's address,
pha ; then replace that address
inc 0,x ; with the variable's actual
bne getval4 ; value before returning
inc 1,x
getval4:
lda (0,x)
sta 1,x ; store high-byte of term value
pla
getval5:
sta 0,x ; store low-byte of term value
getrts:
rts
;-----------------------------------------------------;
; Apply the binary operator in a to var[x] and var[x+2]
; Valid VTL02B operators are {+ - * / & | ^ < = >}
; {>} is defined as greater than _or_equal_
; An undefined operator will be interpreted as one of
; the comparison operators
; 190 bytes
oper:
cmp #'*' ; multiplication operator?
bne oper2 ; no: next case
; - - - - - - - - - - - - - - - - - - - - - - - - - - ;
; 16-bit unsigned multiply routine
; overflow is ignored/discarded
; var[x] *= var[x+2], var[x+2] = 0, {>} is modified
;
mul:
lda 0,x
sta gthan
lda 1,x ; {>} = var[x]
sta gthan+1
lda #0
sta 0,x ; var[x] = 0
sta 1,x
mul2:
lsr gthan+1
ror gthan ; {>} /= 2
bcc mul3
jsr plus ; form the product in var[x]
mul3:
asl 2,x
rol 3,x ; left-shift var[x+2]
lda 2,x
ora 3,x ; loop until var[x+2] = 0
bne mul2
rts
oper2:
cmp #'/' ; division operator?
bne oper3 ; no: next case
; - - - - - - - - - - - - - - - - - - - - - - - - - - ;
; 16-bit unsigned division routine
; var[x] /= var[x+2], {%} = remainder, {>} modified
; var[x] /= 0 produces {%} = var[x], var[x] = 65535
;
div:
lda #0
sta remn ; {%} = 0
sta remn+1
lda #16
sta gthan ; {>} = loop counter
div1:
asl 0,x ; var[x] is gradually replaced
rol 1,x ; with the quotient
rol remn ; {%} is gradually replaced
rol remn+1 ; with the remainder
lda remn
cmp 2,x
lda remn+1 ; partial remainder >= var[x+2]?
sbc 3,x
bcc div2
sta remn+1 ; yes: update the partial
lda remn ; remainder and set the
sbc 2,x ; low bit in the partial
sta remn ; quotient
inc 0,x
div2:
dec gthan
bne div1 ; loop 16 times
rts
oper3:
cmp #'+' ; addition operator?
bne oper4 ; no: next case
; - - - - - - - - - - - - - - - - - - - - - - - - - - ;
plus:
clc ; var[x] += var[x+2]
dex
jsr plus2
inx
plus2:
lda 1,x
adc 3,x
sta 1,x
rts
oper4:
dex ; (factored from the following ops)
cmp #'-' ; subtraction operator?
bne oper5 ; no: next case
; - - - - - - - - - - - - - - - - - - - - - - - - - - ;
minus:
sec ; var[x] -= var[x+2]
jsr minus2
inx
minus2:
lda 1,x
sbc 3,x
sta 1,x
rts
oper5:
cmp #'&' ; bit-wise and operator?
bne oper6 ; no: next case
; - - - - - - - - - - - - - - - - - - - - - - - - - - ;
jsr and_2 ; var[x] &= var[x+2]
inx
and_2:
lda 1,x
and 3,x
bcs oper8e ; (always taken)
oper6:
cmp #OP_OR ; bit-wise or operator?
bne oper7 ; no: next case
; - - - - - - - - - - - - - - - - - - - - - - - - - - ;
jsr or_2 ; var[x] |= var[x+2]
inx
or_2:
lda 1,x
ora 3,x
bcs oper8e ; (always taken)
oper7:
cmp #'^' ; bit-wise xor operator?
bne oper8 ; no: next case
; - - - - - - - - - - - - - - - - - - - - - - - - - - ;
jsr xor_2 ; var[x] ^= var[x+2]
inx
xor_2:
lda 1,x
eor 3,x
bcs oper8e ; (always taken)
;-----------------------------------------------------;
; Apply comparison operator in a to var[x] and var[x+2]
; and place result in var[x] (1: true, 0: false)
;
oper8:
eor #'<' ; 0: '<' 1: '=' 2: '>'
sta gthan ; Other values in a are undefined,
jsr minus ; but _will_ produce some result
dec gthan ; var[x] -= var[x+2]
bne oper8b ; equality test?
ora 0,x ; yes: 'or' high and low bytes
beq oper8c ; (cs) if 0
clc ; (cc) if not 0
oper8b:
lda gthan
rol
oper8c:
adc #0
and #1 ; var[x] = 1 (true), 0 (false)
oper8d:
sta 0,x ; var[x] -> simple variable
lda #0
oper8e:
sta 1,x
rts
;-----------------------------------------------------;
; Set var[x] to the address of the variable named in a
; entry: a holds variable name, @[y] -> text holding
; array index expression (if a = ':')
; uses: plus, eval, oper8d, {@ &}
; exit: (eq): var[x] -> variable, @[y] unchanged
; (ne): var[x] -> array element,
; @[y] -> following text
; 26 bytes
convp:
cmp #':' ; array element?
bne simple ; no: var[x] -> simple variable
jsr eval ; yes: evaluate array index at
asl 0,x ; @[y] and advance y
rol 1,x
lda ampr ; var[x] -> array element
sta 2,x ; at address 2*index+&
lda ampr+1
sta 3,x
bne plus ; (always taken)
; The following section is designed to translate the
; named simple variable from its ASCII value to its
; zero-page address. In this case, 'A' translates
; to $82, '!' translates to $c2, etc. The method
; employed must correspond to the zero-page equates
; above, or strange and not-so-wonderful bugs will
; befall the weary traveller on his or her porting
; journey.
simple:
asl ; form simple variable address
ora #$80 ; mapping function is (a*2)|128
bmi oper8d ; (always taken)
;-----------------------------------------------------;
; If text at @[y] is a decimal constant, translate it
; into var[x] (discarding any overflow) and update y
; entry: @[y] -> text containing possible constant
; Leading space characters are skipped, but
; any spaces encountered after a conversion
; has begun will end the conversion.
; uses: mul, plus, var[x], var[x+2], {@ > ?}
; exit: (ne): var[x] = constant, @[y] -> next text
; (eq): var[x] = 0, @[y] unchanged
; (cs): in all but the truly strangest cases
; 41 bytes
cvbin:
lda #0
sta 0,x ; var[x] = 0
sta 1,x
sta 3,x
jsr getbyte ; skip any leading spaces
sty ques ; save pointer
cvbin2:
lda (at),y ; grab a char
eor #$30 ; if char at @[y] is not a
cmp #10 ; decimal digit then stop
bcs cvbin3 ; the conversion
pha ; save decimal digit
lda #10
sta 2,x
jsr mul ; var[x] *= 10
pla ; retrieve decimal digit
sta 2,x
jsr plus ; var[x] += digit
iny ; loop for more digits
bpl cvbin2 ; (with safety escape)
cvbin3:
cpy ques ; (ne) if valid, (eq) if not
rts
;-----------------------------------------------------;
; Accept input line from user and store it in linbuf,
; zero-terminated (allows very primitive edit/cancel)
; entry: (jsr to inln or newln, not inln6)
; uses: linbuf, inch, outcr, {@}
; exit: @[y] -> linbuf
; 42 bytes
inln6:
cmp #ESC ; escape?
beq newln ; yes: discard entire line
iny ; line limit exceeded?
bpl inln2 ; no: keep going
newln:
jsr outnl ; yes: discard entire line
inln:
ldy #linbuf ; entry point: start a fresh line
sty at ; {@} -> input line buffer
ldy /linbuf
sty at+1
ldy #1
inln5:
dey
bmi newln
inln2:
jsr inch ; get (and echo) one key press
cmp #BS ; backspace?
beq inln5 ; yes: delete previous char
cmp #$0d ; cr?
bne inln3
lda #0 ; yes: replace with null
inln3:
sta (at),y ; put key in linbuf
bne inln6 ; continue if not null
tay ; y = 0
rts
;-----------------------------------------------------;
; Find the first/next stored program line >= {#}
; entry: (cc): start search at program beginning
; (cs): start search at next line after {@}
; uses: prgm, {@ # & (}
; exit: (cs): {@} >= {&}, {(} = garbage, y = 2
; (cc): {@} -> found line, {(} = actual line
; number, y = 2
; 53 bytes
find:
bcs findnxt ; cs: search begins at next line
lda /prgm ; cc: search begins at first line
sta at+1
lda #prgm ; {@} -> first program line
bcc find1st ; (always taken)
findnxt:
jsr checkat ; if {@} >= {&} then the search
bcs findrts ; failed, so return with (cs)
lda at
adc (at),y ; {@} += length of current line
find1st:
sta at
bcc getlpar
inc at+1
getlpar:
ldy #0
lda (at),y
sta lparen ; {(} = current line number
cmp pound ; (invalid if {@} >= {&}, but
iny ; we'll catch that later...)
lda (at),y
sta lparen+1 ; if {(} < {#} then try the next
sbc pound+1 ; program line
bcc findnxt ; else the search is complete
checkat:
ldy #2
lda at ; {@} >= {&} (end of program)?
cmp ampr
lda at+1 ; yes: search failed (cs)
sbc ampr+1 ; no: clear carry
findrts:
rts
;-----------------------------------------------------;
; Fetch a byte at @[y], ignoring space characters
; 10 bytes
skpbyte:
iny ; skip over current char
getbyte:
lda (at),y
eor #' '
beq skpbyte ; skip over any space char(s)
eor #' ' ; set flags for char loaded
rts
;-----------------------------------------------------;
; Check for user keypress and return with (cc) if none
; is pending. Otherwise, fall through to inch
; and return with (cs).
; 6 bytes
inkey:
lda KBD ; Is there a keypress waiting?
asl
bcc outrts ; no: return with (cc)
; - - - - - - - - - - - - - - - - - - - - - - - - - - ;
; Read key from stdin into a, echo, (cs)
; Dump stack and abort to "OK" prompt if ctrl-C
; 16 bytes
inch:
sty dolr ; save y reg
jsr KEYIN ; get a char from keyboard
ldy dolr ; restore y reg
and #$7f ; strip apple's hi-bit
cmp #$03 ; ctrl-C?
bne outch ; no: echo to terminal
jmp start ; yes: abort to "OK" prompt
; - - - - - - - - - - - - - - - - - - - - - - - - - - ;
; Print ascii char in a to stdout, (cs)
; 9 bytes
outch:
pha ; save original char
ora #$80 ; apples prefer "high" ascii
jsr COUT ; emit char via apple monitor
pla ; restore original char
sec ; (by contract with callers)
outrts:
rts
;-----------------------------------------------------;
.en vtl02ba2
Last page update: October 4, 2015.