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27
CODE.md
27
CODE.md
@ -51,6 +51,25 @@ Therefore, shadow registers should only be used in code that doesn't call
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routines or that call a routine that explicitly states that it preserves
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routines or that call a routine that explicitly states that it preserves
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shadow registers.
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shadow registers.
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Another important note is that routines returning success with Z generally don't
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preserve AF: too complicated. But otherwise, AF is often preserved. For example,
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register fiddling routines in core try to preserve AF.
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## Z for success
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The vast majority of routines use the Z flag to indicate success. When Z is set,
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it indicates success. When Z is unset, it indicates error. This follows the
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tradition of a zero indicating success and a nonzero indicating error.
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Important note: only Z indicate success. Many routines return a meaningful
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nonzero value in A and still set Z to indicate success.
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In error conditions, however, most of the time A is set to an error code.
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In many routines, this is specified verbosely, but it's repeated so often that
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I started writing it in short form, "Z for success", which means what is
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described here.
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## Stack management
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## Stack management
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Keeping the stack "balanced" is a big challenge when writing assembler code.
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Keeping the stack "balanced" is a big challenge when writing assembler code.
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@ -87,4 +106,12 @@ comments. Example:
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pop af ; <-- lvl 1
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pop af ; <-- lvl 1
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I think that this should do the trick, so I'll do this consistently from now on.
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I think that this should do the trick, so I'll do this consistently from now on.
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## String length
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Pretty much every routine expecting a string have no provision for a string
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that doesn't have null termination within 0xff bytes. Treat strings of such
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lengths with extra precaution and distrust proper handling of existing routines
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for those strings.
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[zasm]: ../apps/zasm/README.md
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[zasm]: ../apps/zasm/README.md
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@ -14,3 +14,17 @@ writing from scratch, so here I am, writing from scratch again...
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The biggest challenge here is to extract code from zasm, adapt it to fit BASIC,
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The biggest challenge here is to extract code from zasm, adapt it to fit BASIC,
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not break anything, and have the wisdom to see when copy/pasting is a better
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not break anything, and have the wisdom to see when copy/pasting is a better
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idea.
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idea.
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## Design goal
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The reason for including a BASIC dialect in Collapse OS is to supply some form
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of system administration swiss knife. zasm, ed and the shell can do
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theoretically anything, but some tasks (which are difficult to predict) can
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possibly be overly tedious. One can think, for example, about hardware
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debugging. Poking and peeking around when not sure what we're looking for can
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be a lot more effective with the help of variables, conditions and for-loops in
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an interpreter.
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Because the goal is not to provide a foundation for complex programs, I'm
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planning on intentionally crippling this BASIC dialect for the sake of
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simplicity.
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117
apps/basic/buf.asm
Normal file
117
apps/basic/buf.asm
Normal file
@ -0,0 +1,117 @@
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; *** Consts ***
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; maximum number of lines (line number maximum, however, is always 0xffff)
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.equ BUF_MAXLINES 0x100
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; Size of the string pool
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.equ BUF_POOLSIZE 0x1000
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; *** Variables ***
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; A pointer to the first free line
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.equ BUF_LFREE BUF_RAMSTART
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; A pointer to the first free byte in the pool
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.equ BUF_PFREE @+2
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; The line index. Each record consists of 4 bytes: 2 for line number,
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; 2 for pointer to string in pool. Kept in order of line numbers.
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.equ BUF_LINES @+2
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; The line pool. A list of null terminated strings. BUF_LINES records point
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; to those strings.
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.equ BUF_POOL @+BUF_MAXLINES*4
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.equ BUF_RAMEND @+BUF_POOLSIZE
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bufInit:
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ld hl, BUF_LINES
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ld (BUF_LFREE), hl
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ld hl, BUF_POOL
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ld (BUF_PFREE), hl
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ret
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; Add line at (HL) with line number DE to the buffer. The string at (HL) should
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; not contain the line number prefix or the whitespace between the line number
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; and the comment.
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; Note that an empty string is *not* an error. It will be saved as a line.
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; Z for success.
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; Error conditions are:
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; * not enough space in the pool
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; * not enough space in the line index
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bufAdd:
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exx ; preserve HL and DE
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; First step: do we have index space?
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ld hl, (BUF_LFREE)
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ld de, BUF_POOL
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or a ; reset carry
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sbc hl, de
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exx ; restore
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; no carry? HL >= BUF_POOL, error. Z already unset
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ret nc
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; Second step: see if we're within the pool's bounds
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call strlen
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inc a ; strlen doesn't include line termination
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exx ; preserve HL and DE
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ld hl, (BUF_PFREE)
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call addHL
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ld de, BUF_RAMEND
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sbc hl, de
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exx ; restore
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; no carry? HL >= BUF_RAMEND, error. Z already unset
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ret nc
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; We have enough space.
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; Third step: set line index data
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push de ; --> lvl 1
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push hl ; --> lvl 2
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ld hl, (BUF_LFREE)
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ld (hl), e
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inc hl
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ld (hl), d
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inc hl
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ld de, (BUF_PFREE)
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ld (hl), e
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inc hl
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ld (hl), d
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inc hl
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ld (BUF_LFREE), hl
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pop hl \ push hl ; <-- lvl 2, restore and preserve
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; Fourth step: copy string to pool
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ld de, (BUF_PFREE)
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call strcpyM
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ld (BUF_PFREE), de
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pop hl ; <-- lvl 2
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pop de ; <-- lvl 1
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ret
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; Set IX to point to the beginning of the pool.
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; Z set if (IX) is a valid line, unset if the pool is empty.
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bufFirst:
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ld ix, BUF_LINES
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jp bufEOF
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; Given a valid line record in IX, move IX to the next line.
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; This routine doesn't check that IX is valid. Ensure IX validity before
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; calling.
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bufNext:
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inc ix \ inc ix \ inc ix \ inc ix
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jp bufEOF
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; Returns whether line index at IX is past the end of file, that is,
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; whether IX == (BUF_LFREE)
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; Z is set when not EOF, unset when EOF.
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bufEOF:
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push hl
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push de
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push ix \ pop hl
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ld de, (BUF_LFREE)
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sbc hl, de
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jr z, .empty
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cp a ; ensure Z
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.end:
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pop de
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pop hl
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ret
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.empty:
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call unsetZ
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jr .end
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; Given a line index in (IX), set HL to its associated string pointer.
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bufStr:
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ld l, (ix+2)
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ld h, (ix+3)
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ret
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@ -17,6 +17,8 @@
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.equ EXPR_PARSE parseLiteral
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.equ EXPR_PARSE parseLiteral
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.inc "lib/expr.asm"
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.inc "lib/expr.asm"
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.inc "basic/tok.asm"
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.inc "basic/tok.asm"
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.equ BAS_RAMSTART USER_RAMSTART
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.equ BUF_RAMSTART USER_RAMSTART
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.inc "basic/buf.asm"
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.equ BAS_RAMSTART BUF_RAMEND
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.inc "basic/main.asm"
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.inc "basic/main.asm"
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USER_RAMSTART:
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USER_RAMSTART:
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@ -12,6 +12,7 @@
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; *** Code ***
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; *** Code ***
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basStart:
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basStart:
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ld (BAS_INITSP), sp
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ld (BAS_INITSP), sp
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call bufInit
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xor a
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xor a
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ld hl, .welcome
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ld hl, .welcome
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call printstr
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call printstr
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@ -32,12 +33,13 @@ basPrompt:
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call basDirect
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call basDirect
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jr basPrompt
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jr basPrompt
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.number:
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.number:
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; do nothing for now, we only support direct mode.
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push ix \ pop de
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ld hl, .sNumber
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call toWS
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call basPrintLn
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call rdWS
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call bufAdd
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jp nz, basERR
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call printcrlf
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jr basPrompt
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jr basPrompt
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.sNumber:
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.db "A number!", 0
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.sPrompt:
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.sPrompt:
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.db "> ", 0
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.db "> ", 0
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@ -104,6 +106,26 @@ basBYE:
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.sBye:
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.sBye:
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.db "Goodbye!", 0
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.db "Goodbye!", 0
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basLIST:
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call printcrlf
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call bufFirst
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ret nz
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.loop:
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ld e, (ix)
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ld d, (ix+1)
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ld hl, BAS_SCRATCHPAD
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call fmtDecimal
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call printstr
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ld a, ' '
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call stdioPutC
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call bufStr
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call printstr
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call printcrlf
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call bufNext
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jr z, .loop
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ret
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basPRINT:
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basPRINT:
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call parseExpr
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call parseExpr
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jp nz, basERR
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jp nz, basERR
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@ -116,6 +138,8 @@ basPRINT:
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basCmds1:
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basCmds1:
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.dw basBYE
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.dw basBYE
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.db "bye", 0, 0, 0
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.db "bye", 0, 0, 0
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.dw basLIST
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.db "list", 0, 0
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; statements
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; statements
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basCmds2:
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basCmds2:
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.dw basPRINT
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.dw basPRINT
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@ -46,3 +46,11 @@ fnWSIdx:
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pop bc
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pop bc
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pop hl
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pop hl
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ret
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ret
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; Advance HL to the next whitespace or to the end of string.
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toWS:
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ld a, (hl)
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call isSep
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ret z
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inc hl
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jr toWS
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