From 46d84e91ebe48ade19bd3f5e18360bf853b53968 Mon Sep 17 00:00:00 2001
From: Mirek Kratochvil <exa.exa@gmail.com>
Date: Fri, 4 Aug 2023 20:11:39 +0200
Subject: [PATCH] make a program

---
 exit_123.s | 70 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 uskel.s    |  2 +-
 2 files changed, 71 insertions(+), 1 deletion(-)
 create mode 100644 exit_123.s

diff --git a/exit_123.s b/exit_123.s
new file mode 100644
index 0000000..9fe9264
--- /dev/null
+++ b/exit_123.s
@@ -0,0 +1,70 @@
+
+# || -> cont
+.func main
+	# push a new integer
+	pushq $100
+	pushq $INT_code
+	mov %rsp, %r11 # backup first arg
+
+	# push another new integer
+	pushq $23
+	pushq $INT_code
+	mov %rsp, %r12 # backup second arg
+
+	# push the plus
+	push %r12
+	push %r11
+	pushq $2
+	pushq $plus
+	mov %rsp, %r13 # backup plus
+
+	# push a cont thunk for main_exit
+	push %rsi
+	pushq $1
+	pushq $main_exit
+
+	# evaluate into main_exit
+	mov %rsp, %rsi
+	enter %r13
+
+# exitcode -> | cont (unused, should be 0) |
+.func main_exit
+	mov 0x8(%rsi), %rdi # result to syscall exitcode
+	mov $0x3c, %rax  # syscall 60
+	syscall # exit %rdi
+
+# | arg1 | arg2 | -> cont
+.func plus
+	# push a thunk for finishing the plus
+	push %rsi # cont
+	push %rbp # ret (self)
+	mov 0x18(%rbp), %rax
+	push %rax # arg2
+	pushq $3
+	pushq $plus_step1
+
+	mov %rsp, %rsi # continue to the new thunk
+	enter 0x10(%rbp) # evaluate arg1
+
+# arg1 -> | arg2 | ret | cont |
+.func plus_step1
+	# this is guaranteed to be entered only once (it's a cont), so we can rewrite the thunk in place
+	mov 0x10(%rbp), %rax
+	movq $plus_fini, 0x00(%rbp)
+	mov %rsi, 0x10(%rbp)
+
+	mov %rbp, %rsi # continue on the rewritten thunk
+	enter %rax # evaluate arg1
+
+# arg2 -> | arg1 | ret | cont |
+.func plus_fini
+	mov 0x8(%rsi), %rax # arg 2 
+	mov 0x10(%rbp), %rsi # location of arg1
+	add 0x8(%rsi), %rax # arg 1
+
+	mov 0x18(%rbp), %rsi # save result to the original plus thunk
+	mov %rax, 0x08(%rsi)
+	movq $INT_code, 0x00(%rsi)
+
+	enter 0x20(%rbp) # eval cont, returning %rsi
+
diff --git a/uskel.s b/uskel.s
index dad9bc5..f0978fc 100644
--- a/uskel.s
+++ b/uskel.s
@@ -8,7 +8,7 @@ _start:
 
 .section .bss
 _memory_state:
-	cell 0 # next free position
+	cell 0 # bottom of allocation (grows down)
 	cell 0 # region start
 	cell 0 # region end