diff --git a/apps/README.md b/apps/README.md
index c85b5c9..6be1999 100644
--- a/apps/README.md
+++ b/apps/README.md
@@ -104,3 +104,8 @@ Symbols have a different meaning depending on the application. In zasm, it's
 labels and constants. In basic, it's variables.
 
 Expressions can't contain spaces.
+
+Expressions can have an empty left operand. It will then be considered as 0.
+This allows signed integers, for example, `-42` to be expressed as expected.
+That form doesn't work well everywhere and is mostly supported for BASIC. In
+zasm, you're safer with `0-42`.
diff --git a/apps/basic/README.md b/apps/basic/README.md
index c3f092a..3848591 100644
--- a/apps/basic/README.md
+++ b/apps/basic/README.md
@@ -47,10 +47,12 @@ by typing a whitespace.
 
 ### Numbers, expressions and variables
 
-Only 16-bit integers (unsigned for now) are supported in this BASIC. When
-printed, they're printed in decimal form. When expressing number literals, you
-can do so either in multiple forms. . See "Number literals" in `apps/README.md`
-for details.
+Numbers are stored in memory as 16-bit integers (little endian) and numbers
+being represented by BASIC are expressed as signed integers, in decimal form.
+Line numbers, however, are expressed and treated as unsigned integers: You can,
+if you want, put something on line "-1", but it will be the equivalent of line
+65535. When expressing number literals, you can do so either in multiple forms.
+See "Number literals" in `apps/README.md` for details.
 
 Expressions are accepted wherever a number is expected. For example,
 `print 2+3` will print `5`.  See "Expressions" in `apps/README.md`.
diff --git a/apps/basic/main.asm b/apps/basic/main.asm
index 1ef9537..800e88b 100644
--- a/apps/basic/main.asm
+++ b/apps/basic/main.asm
@@ -192,7 +192,7 @@ basPRINT:
 	jr	nz, .parseError
 	push	ix \ pop de
 	ld	hl, SCRATCHPAD
-	call	fmtDecimal
+	call	fmtDecimalS
 	call	printstr
 	pop	hl		; <-- lvl 1
 .chkAnother:
diff --git a/apps/lib/expr.asm b/apps/lib/expr.asm
index 53d941d..57eb33d 100644
--- a/apps/lib/expr.asm
+++ b/apps/lib/expr.asm
@@ -97,8 +97,16 @@ _findAndSplit:
 ; parse expression on the left (HL) and the right (DE) and put the results in
 ; HL (left) and DE (right)
 _resolveLeftAndRight:
+	; special case: is (HL) zero? If yes, it means that our left operand
+	; is empty. consider it as 0
+	ld	ix, 0		; pre-set to 0
+	ld	a, (hl)
+	or	a
+	jr	z, .skip
+	; Parse left operand in (HL)
 	call	parseExpr
 	ret	nz		; return immediately if error
+.skip:
 	; Now we have parsed everything to the left and we have its result in
 	; IX. What we need to do now is the same thing on (DE) and then apply
 	; the + operator. Let's save IX somewhere and parse this.
diff --git a/apps/lib/fmt.asm b/apps/lib/fmt.asm
index 25e62b9..74236ed 100644
--- a/apps/lib/fmt.asm
+++ b/apps/lib/fmt.asm
@@ -1,4 +1,25 @@
 
+; Same as fmtDecimal, but DE is considered a signed number
+fmtDecimalS:
+	bit	7, d
+	jr	z, fmtDecimal	; unset, not negative
+	; Invert DE. spit '-', unset bit, then call fmtDecimal
+	push	de
+	ld	a, '-'
+	ld	(hl), a
+	inc	hl
+	ld	a, d
+	cpl
+	ld	d, a
+	ld	a, e
+	cpl
+	ld	e, a
+	inc	de
+	call	fmtDecimal
+	dec	hl
+	pop	de
+	ret
+
 ; Format the number in DE into the string at (HL) in a decimal form.
 ; Null-terminated. DE is considered an unsigned number.
 fmtDecimal:
diff --git a/tools/tests/unit/test_expr.asm b/tools/tests/unit/test_expr.asm
index b4cdc1c..e52ab25 100644
--- a/tools/tests/unit/test_expr.asm
+++ b/tools/tests/unit/test_expr.asm
@@ -149,6 +149,8 @@ testParseExpr:
 	call	.testEQ
 	ld	iy, .t7
 	call	.testEQ
+	ld	iy, .t8
+	call	.testEQ
 	ret
 
 .testEQ:
@@ -186,6 +188,9 @@ testParseExpr:
 .t7:
 	.dw	0xcfb8
 	.db	"0x99f7{3", 0
+.t8:
+	.dw	0xffff
+	.db	"-1", 0
 
 nexttest:
 	ld	a, (testNum)
diff --git a/tools/tests/unit/test_lib_fmt.asm b/tools/tests/unit/test_lib_fmt.asm
index 049717f..54a05ff 100644
--- a/tools/tests/unit/test_lib_fmt.asm
+++ b/tools/tests/unit/test_lib_fmt.asm
@@ -11,6 +11,7 @@ test:
 	ld	sp, 0xffff
 
 	call	testFmtDecimal
+	call	testFmtDecimalS
 
 	; success
 	xor	a
@@ -55,6 +56,30 @@ testFmtDecimal:
 	.dw 0xffff
 	.db "65535", 0
 
+testFmtDecimalS:
+	ld	ix, .t1
+	call	.test
+	ld	ix, .t2
+	call	.test
+	ret
+.test:
+	ld	e, (ix)
+	ld	d, (ix+1)
+	ld	hl, sandbox
+	call	fmtDecimalS
+	ld	hl, sandbox
+	push	ix \ pop de
+	inc	de \ inc de
+	call	strcmp
+	jp	nz, fail
+	jp	nexttest
+.t1:
+	.dw 1234
+	.db "1234", 0
+.t2:
+	.dw 0-1234
+	.db "-1234", 0
+
 nexttest:
 	ld	a, (testNum)
 	inc	a