zasm: add "last value" symbol (@)

apps/zasm/README.md

@@ -1,8 +1,7 @@
 # z80 assembler
 
-This is probably the most critical part of the Collapse OS project. If this app
-can be brought to completion, it pretty much makes the project a success because
-it ensures self-reproduction.
+This is probably the most critical part of the Collapse OS project because it
+ensures its self-reproduction.
 
 ## Running on a "modern" machine
 
@@ -92,6 +91,12 @@ The `$` is a special symbol that can be placed in any expression and evaluated
 as the current output offset. That is, it's the value that a label would have if
 it was placed there.
 
+## The Last Value
+
+Whenever a `.equ` directive is evaluated, its resulting value is saved in a
+special "last value" register that can then be used in any expression. This
+is very useful for variable definitions and for jump tables.
+
 ## Includes
 
 The `#inc` directive is special. It takes a string literal as an argument and

apps/zasm/directive.asm

@@ -11,7 +11,9 @@
 .equ	D_BAD	0xff
 
 ; *** Variables ***
-.equ	DIREC_SCRATCHPAD	DIREC_RAMSTART
+; Result of the last .equ evaluation. Used for "@" symbol.
+.equ	DIREC_LASTVAL		DIREC_RAMSTART
+.equ	DIREC_SCRATCHPAD	DIREC_LASTVAL+2
 .equ	DIREC_RAMEND		DIREC_SCRATCHPAD+SCRATCHPAD_SIZE
 ; *** CODE ***
 
@@ -148,6 +150,8 @@ handleEQU:
 	jr	nz, .badarg
 	ld	hl, DIREC_SCRATCHPAD
 	push	ix \ pop de
+	; Save value in "@" special variable
+	ld	(DIREC_LASTVAL), de
 	call	symRegisterConst	; A and Z set
 	jr	z, .end			; success
 	; register ended up in error. We need to figure which error. If it's

apps/zasm/glue.asm

@@ -76,12 +76,12 @@ jp	zasmMain
 .equ	TOK_RAMSTART	IO_RAMEND
 #include "zasm/tok.asm"
 #include "lib/parse.asm"
-#include "zasm/parse.asm"
-#include "zasm/expr.asm"
 .equ	INS_RAMSTART	TOK_RAMEND
 #include "zasm/instr.asm"
 .equ	DIREC_RAMSTART	INS_RAMEND
 #include "zasm/directive.asm"
+#include "zasm/parse.asm"
+#include "zasm/expr.asm"
 .equ	SYM_RAMSTART	DIREC_RAMEND
 #include "zasm/symbol.asm"
 .equ	ZASM_RAMSTART	SYM_RAMEND

apps/zasm/parse.asm

@@ -168,13 +168,19 @@ parseLiteral:
 parseNumberOrSymbol:
 	call	parseLiteral
 	ret	z
-	; Not a number. Try PC
-	push	de		; --> lvl 1
-	ld	de, .sDollar
-	call	strcmp
-	pop	de		; <-- lvl 1
+	; Not a number.
+	; Is str a single char? If yes, maybe it's a special symbol.
+	call	strIs1L
+	jr	nz, .symbol	; nope
+	ld	a, (hl)
+	cp	'$'
 	jr	z, .returnPC
-	; Not PC either, try symbol
+	cp	'@'
+	jr	nz, .symbol
+	; last val
+	ld	ix, (DIREC_LASTVAL)
+	ret
+.symbol:
 	push	de		; --> lvl 1
 	call	symFindVal	; --> DE
 	jr	nz, .notfound
@@ -197,5 +203,3 @@ parseNumberOrSymbol:
 	push	hl \ pop ix
 	pop	hl
 	ret
-.sDollar:
-	.db	'$', 0

apps/zasm/util.asm

@@ -52,6 +52,16 @@ strlen:
 	pop	bc
 	ret
 
+; Sets Z if string at (HL) is one character long
+strIs1L:
+	xor	a
+	cp	(hl)
+	jp	z, unsetZ	; empty string
+	inc	hl
+	cp	(hl)		; Z has proper value
+	dec	hl		; doesn't touch Z
+	ret
+
 ; Compares strings pointed to by HL and DE up to A count of characters in a
 ; case-insensitive manner.
 ; If equal, Z is set. If not equal, Z is reset.

tools/emul/Makefile

@@ -43,15 +43,6 @@ updatebootstrap: $(ZASMBIN) $(INCCFS)
 	$(ZASMSH) $(KERNEL) < zasm/glue.asm > zasm/kernel.bin
 	$(ZASMSH) $(KERNEL) $(APPS) zasm/user.h < $(APPS)/zasm/glue.asm > zasm/zasm.bin
 
-# Sometimes, when developing zasm, stuff get messed up and it's hard to unmess
-# because zasm's brake-up ends up in its bootstrap bins. Sure, we can revert
-# from git, but if we're in the middle of some work, it's inconvenient. As long
-# as we don't diverge from scas's syntax, it can come to the recue!
-.PHONY: rescue
-rescue:
-	scas -o zasm/kernel.bin -I $(KERNEL) zasm/glue.asm
-	scas -o zasm/zasm.bin -I $(APPS) -I $(KERNEL) -I zasm $(APPS)/zasm/glue.asm
-
 .PHONY: clean
 clean:
 	rm -f $(TARGETS) $(SHELLAPPS) {zasm,shell}/*-bin.h

tools/tests/unit/test_expr.asm

@@ -4,6 +4,9 @@
 .equ	ZASM_REG_BUFSZ		0x1000
 .equ	ZASM_LREG_BUFSZ		0x200
 
+; declare DIREC_LASTVAL manually so that we don't have to include directive.asm
+.equ	DIREC_LASTVAL	RAMSTART
+
 jp	test
 
 #include "core.asm"
@@ -13,7 +16,7 @@ jp	test
 #include "zasm/const.asm"
 #include "lib/parse.asm"
 #include "zasm/parse.asm"
-.equ	SYM_RAMSTART	RAMSTART
+.equ	SYM_RAMSTART	DIREC_LASTVAL+2
 #include "zasm/symbol.asm"
 #include "zasm/expr.asm"
 

tools/tests/unit/test_parse_z.asm

@@ -1,3 +1,7 @@
+.equ	RAMSTART	0x4000
+; declare DIREC_LASTVAL manually so that we don't have to include directive.asm
+.equ	DIREC_LASTVAL	RAMSTART
+
 jp	test
 
 #include "core.asm"
@@ -80,6 +84,13 @@ test:
 	jp	nz, fail
 	call	nexttest
 
+.equ	FOO		0x42
+.equ	BAR		@+1
+	ld	a, BAR
+	cp	0x43
+	jp	nz, fail
+	call	nexttest
+
 	; success
 	xor	a
 	halt