;
; BBCBasic for the pdp11 
; (C)J.G.Harston 1988
; v0.00 22/03/89 Started writing
; v0.10 24/03/89 Displays message, gets to prompt
;
; NB: mov label,reg   - relative value of label
;     mov @#label,reg - absolute value of label

org &0			; Relocatable code
;
; Code entered with r0-r5=0, r6=top of memory
br  basic_startup	; Branch to start code
halt			; some padding for future
halt
halt
halt
;
.startup_message
EQUS "PDP11 BBC BASIC v0.10":EQUB 10:EQUB 13
EQUS "(C) Copyright J.G.Harston 1989":EQUB 10:EQUB 13
EQUB 0
align
:
.basic_startup
; should check if filename passed to CHAIN
; don't print startup message if CHAINing
mov startup_message,r1
jsr pc,print_r1
; bottom of memory is end of code+system variables
; top of memory is passed in r6
mov end_of_basic,r0	; r0=bottom of memory
mov sp,r1		; r1=top of memory
mov r0,SV_PAGE		; initialise bottom and
mov r1,SV_MEMTOP	; and top of memory
mov r1,sp		; put stack at top of memory
;
;
.immediate_loop
mov #ASC">",r0
jsr pc,IO_WRCH		; print ">" prompt
mov CMD_BUF,r1
jsr pc,IO_READLINE	; Read a line of input
;
; tokenise it
;
;
; mov immediate_loop,pc	; jump back to command prompt
BR immediate_loop takes ; 2 bytes less
;

; TOKEN TABLE
EQUB &80:EQUS "AND"
EQUB &81:EQUS "etc"
:
EQUB &FF:EQUS "OSCLI"
EQUB &FF
align
halt
:
.print_r1		; Print text pointed to by R1, term. by &00
mov r0,-(sp)		; push r0
.print_r1_loop
movb (r1)+,r0		; get byte from r1, inc r1
beq print_r1_end	; exit if final byte
jsr pc,IO_WRCH
br print_r1_loop	; loop back
.print_r1_end
mov (sp)+,r0		; pop r0
rts pc			; and exit
;



; IO ROUTINES
;
; ***** GENERAL I/O *****
;
.IO_READLINE
; R1=>memory to read input string to
; In future, max/lochar/hichar
mov  r1,-(sp)			; Save pointer to buffer
mov  #0,r2			; Zero number of characters read
.IO_RDLNloop
jsr  pc,IO_RDCH		; Get a character
bcs  IO_RDLNesc		; Escape state
cmp  r0,#27;beq IO_RDLNesc	; Escape character (remove later)
cmp  r0,#13:beq IO_RDLNcr	; <CR> - End of line
cmp  r0,#10:beq IO_RDLNcr	; <LF> - End of line
cmp  r0,#21:beq IO_RDLNu	; Ctrl-U - delete line
cmp  r0,#127:beq IO_RDLNdel	; <DEL> - del a character
cmp  r0,#8:beq IO_RDLNdel	; <BS> - del a character
cmp  r0,#ASC" ":bcs IO_RDLNloop; Ignore other control characters
cmp  r2,#240:bcc IO_RDLNloop	; No more room for characters
movb r0,(r1)+			; Put character into memory
add  r2,#1			; Inc. number of characters
jsr  pc,IO_WRCH		; Output the character
br   IO_RDLNloop		; Go back for another
;
.IO_RDLNu
mov  r2,r3			; We want to delete all characters
br   IO_RDLNdelete
.IO_RDLNdel
mov  #1,r3			; We only want to delete one char
.IO_RDLNdelete
cmp  r2,#0:beq IO_RDLNloop
mov #127,r0
jsr  pc,IO_WRCH		; Output <DEL> character
sub  r1,r1,#1			; Back address pointer
sub  r2,r2,#1			; Dec character counter
sub  r3,r3,#1			; Dec number to delete
bne  IO_RDLNdelete		; Delete more
br   IO_RDLNloop		; Go back into ReadLine loop
;
.IO_RDLNesc
jsr  pc,IO_NEWL		; Print newline
mov  #0,r2			; Length = 0
sec				; Set carry - escape
bcs IO_RDLNend
;
.IO_RDLNcr
jsr  pc,IO_NEWL		; Returns with r0=<cr>
movb r0,(r1)+			; Put <CR> terminator in
clc				; Clear carry - ok
.IO_RDLNend
mov  (sp)+,r1			; Get buffer address back
rts  pc			; r0=<cr>, r1=buffer, r2=length
;
;
; ***** HOST I/O *****
;
.IO_QUIT
emt 0
rts pc			; Won't actually get back!
;
.IO_CLI
emt 1			; r1=>command string
rts pc
;
.IO_BYTE
emt 2			; Osbyte r0,r1,r2
rts pc
;
.IO_WORD
emt 3			; Osword r0,r1=>block
rts pc
;
.IO_WRCH
emt 4			; Oswrch r0=char
rts pc
;
.IO_RDCH
emt 5			; Osrdch r0=char
rts pc
;
; Filing entries, etc.
;
;
; System Variables
; Memory locations, increasing address
.SV_MEMBOT	equw &0000	; Bottom of available memory
.SV_PAGE	equw &0000	; PAGE - start of Basic program
.SV_TOP	equw &0000	; End of Basic program
.SV_LOMEM	equw &0000	; Start of Basic variable area
.SV_VAREND	equw &0000	; End of Basic variable area
.SV_STACK	equw &0000	; Bottom of Basic stack
.SV_HIMEM	equw &0000	; Top of Basic stack/memory
.SV_MEMTOP	equw &0000	; Top of available memory
;
; Interpreting pointers
.SV_PTR	equw &0000	; Current byte being executed
;
; Buffers
.STRING_BUF	equm 256	; String accumulator
.CMD_BUF	equm 256	; Command Input buffer
.tempstack			; Temporary startup stack in command buffer
;
; Default PAGE:
.end_of_basic