2 files changed, 19 insertions(+), 3 deletions(-)

M README.md
M src/main.s

@@ 10,6 10,7 @@ implements only the surface functionality though - decimal integers and
 
 * `.` - print last value in the stack
 * `d` - drop last value from the stack
+* `s` - swap two last values
 
 Interactions are done through the default input device, in case of most systems,
 that would be serial. Entering a number to push it onto the stack works as

@@ 29,6 29,8 @@ kmain:
 	li t0, 7
 	beq a0, t0, exec_drop
 	li t0, 8
+	beq a0, t0, exec_swap
+	li t0, 9
 	beq a0, t0, exec_invalid
 exec_number:
 	add sp, sp, -8


@@ 107,12 109,19 @@ exec_drop:
 	li a0, 'd'
 	call printcln
 	j 2f
+exec_swap:
+	ld t0, (sp)
+	ld t1, 8(sp)
+	sd t1, (sp)
+	sd t0, 8(sp)
+	li a0, 's'
+	call printcln
+	j 2f
 exec_invalid:
 	li a0, '?'
 	call printcln
 2:
 	j 1b
-
 3:
 	add sp, fp, 16
 	ld ra, 8(fp)


@@ 129,7 138,8 @@ exec_invalid:
         | 5 (remainder)
         | 6 (print)
         | 7 (drop)
-        | 8 (invalid)
+        | 8 (swap)
+        | 9 (invalid)
 */
 readatom:
 	sd fp, -16(sp)


@@ 163,6 173,8 @@ retry:
 	beq a0, t0, printstack
 	li t0, 'd'
 	beq a0, t0, drop
+	li t0, 's'
+	beq a0, t0, swap
 
 	/* 0 < (a0 - '0') < 9 */
 	add a0, a0, -'0'


@@ 216,8 228,11 @@ printstack:
 drop:
 	li a0, 7
 	j 2f
-invalid:
+swap:
 	li a0, 8
+	j 2f
+invalid:
+	li a0, 9
 2:
 	add sp, fp, 16
 	ld ra, 8(fp)