diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index ed897a3..b50237b 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -16,10 +16,22 @@ All functions in the standard library should follow the following outline:
 ; Arguments: <which register contains what argument>
 ; Result: <what is the result, and where is it stored>
 ; Clobbers: <list of registers that are clobbered>
+; Globals: <list of globals are accessed. OPTIONAL>
+; Errors: <list of any status codes returned in flags. OPTIONAL>
 <label>: <;SHOULD BE INLINED>
     <CODE>
 
 ```
 
 Functions should be in the appropriate asm file, if you are unsure where functionality fits make a seperate file and ask in the pull request
-Functions that are provided for convenience/reference but should be inlined in production code should be marked with `;SHOULD BE INLINED` after their label
+Functions that are provided for convenience/reference but should be inlined in production code should be marked with `;SHOULD BE INLINED` after their label.
+
+If the function returns a status code in flags, the preamble should list all it might return.
+
+## Globals
+
+Any function that uses global variables should document this in the preamble comment, see above.
+
+A file/module should check on intialisation that the address `globals.MAGIC_ADDRESS` contains the 16 bit value `globals.MAGIC_VALUE`, to ensure that the user has properly included [[src/start.asm]] and reservered the global variable area.
+
+No opinion is offered on whether modules can assume globals variables are initialised to zero.
\ No newline at end of file
diff --git a/README.md b/README.md
index b973fa2..dcd9ff3 100644
--- a/README.md
+++ b/README.md
@@ -3,7 +3,7 @@
 This is a standard library for symphony.
 It is both intended as a practical toolkit to develop more complex software as well as a teaching resource.
 
-If you just want to use the standard library [[src/stdlib.asm]] is your main header, include it after your code.
+If you just want to use the standard library [[src/stdlib.asm]] is your main header, include it after your code. Some modules also require [[src/start.asm]] at the very start of the program.
 
 If you are using it as a learning resource have a look at the [teaching folder](teaching).
 
@@ -20,7 +20,7 @@ If you are intersted in contributing have a look at [[CONTRIBUTING.md]]
 | preserved | sp, r8 - r12 |
 | scratch | flags, r1 - r7 |
 | arguments | r1 - r7 |
-| result | r1-r7 |
+| result | flags, r1 - r7 |
 | return address | r13 |
 
 Arguments not fitting into the 7 registers should be passed on the top of the stack, meaning they should be the last values pushed before the function call.
@@ -32,9 +32,14 @@ Should a function return more values than fit into the 7 registers, the caller h
 This reduces the amount of argument registers to 6 and all arguments above that shall go on the stack, the pointer to the result stack shall **always** be passed in an argument register.
 This register points at the highest available address for results, with the 8th result being stored there, the 9th below it and so on.
 
+Some functions return a success/failure status code in flags. A set low bit will indicate some error condition, the function may return more than one possible value to report different errors. A flags value of 0 is set on success. No other even values are used. This can be checked with `je` or `jne` immediatly on return to the caller. All other functions clobber flags.
+
 ### Stack
 Grows downwards from 0xXXFE_0000 (so top of memory -0x1_0000).
 
+### Globals
+Global variables are stored near the bottom of RAM in the address range 0x0010..0x0100. Programs should `include src/start` as the first line before their own code and before other includes, or otherwise reserve this space, the first instruction should be a jump to user code.
+
 ### Types
 
 #### String
diff --git a/isa/README.md b/isa/README.md
new file mode 100644
index 0000000..592b680
--- /dev/null
+++ b/isa/README.md
@@ -0,0 +1,43 @@
+Symphony ISA specs for use in stdlib
+
+# `default.isa`
+The stock game ISA. Production code must comply to this. Taken from `symphony_11_functions` in game version 2.1.330A
+
+# `test.isa`
+ISA with additional instructions. These additional instructions may be used in
+the tests directory only. Some of these instructions require hardware changes.
+
+## Labels in more instructions
+
+`cmp`, `load`, and `store` instructions (but not `pload` and `pstore`) now also accept labels.
+
+## `qcall %a(label)`
+Performs a jump-and-link function call, with label size up to 32 bits.
+
+_HARDWARE:_ Store PC+4 in the result register when in jump mode.
+
+## `li %a(register) %b:U32(immediate | label)`
+Loads an immediate up to 32 bits long
+
+_HARDWARE:_ No requirements
+
+## `halt%c(condition)`
+Halts if `flags` matches the condition.
+
+_HARDWARE:_ Halt if instruction bit 31 is set AND the condition block is true.
+
+```
+condition
+""     1000
+".all" 1000
+".e"   0001
+".ne"  1001
+".b"   0010
+".ae"  1010
+".be"  0011
+".a"   1011
+".l"   0100
+".ge"  1100
+".le"  0101
+".g"   1101
+```
diff --git a/isa/default.isa b/isa/default.isa
new file mode 100644
index 0000000..f9514c8
--- /dev/null
+++ b/isa/default.isa
@@ -0,0 +1,316 @@
+[settings]
+name = "Symphony"
+
+[fields]
+
+register
+zr 0000
+r1 0001
+r2 0010
+r3 0011
+r4 0100
+r5 0101
+r6 0110
+r7 0111
+r8 1000
+r9 1001
+r10 1010
+r11 1011
+r12 1100
+r13 1101
+sp 1110
+flags 1111
+
+[instructions]
+
+nop
+00000000 00000000 00000000 00000000
+# (31337_88272577071256, `Does nothing.`)
+
+in %a(register)
+00000001 aaaa0000 00000000 00000000
+# (31337_44524771082050, `Loads an input and stores it in %a.`)
+
+out %b(register)
+00000010 00000000 0000bbbb 00000000
+# (31337_18254089726214, `Sends %b to output.`)
+
+out %a:U16(immediate)
+00010010 00000000 aaaaaaaa aaaaaaaa
+# (31337_15915531259935, `Sends %a to output.`)
+
+keyboard %a(register)
+00000011 aaaa0000 00000000 00000000
+# (31337_27868418224252, `Loads keyboard input and stores it to %a.`)
+
+screen %a(register), %b(register)
+00000100 0000aaaa 0000bbbb 00000000
+# (31337_39857449355186, `Stores the value of %b in screen setting %a.`)
+
+screen %a(register), %b:U16(immediate)
+00010100 0000aaaa bbbbbbbb bbbbbbbb
+# (31337_12835125621741, `Stores the value of %b in screen setting %a.`)
+
+time_0 %a(register)
+00000101 aaaa0000 00000000 00000000
+# (31337_72551400437977, `Loads the lower four bytes of the time value and stores it to %a.`)
+
+time_1 %a(register)
+00000110 aaaa0000 00000000 00000000
+# (31337_68219187873304, `Loads the upper four bytes of the time value and stores it to %a.`)
+
+counter %a(register)
+00000111 aaaa0000 00000000 00000000
+# (31337_67376584796076, `Loads the counter value and stores it to %a.`)
+
+
+
+nand %a(register), %b(register), %c(register)
+00100000 aaaabbbb 0000cccc 00000000
+# (31337_86705346481666, `NAND %b and %c and stores the result in %a.`)
+
+or %a(register), %b(register), %c(register)
+00100001 aaaabbbb 0000cccc 00000000
+# (31337_40732211165530, `OR %b and %c and stores the result in %a.`)
+
+and %a(register), %b(register), %c(register)
+00100010 aaaabbbb 0000cccc 00000000
+# (31337_85679146831122, `AND %b and %c and stores the result in %a.`)
+
+nor %a(register), %b(register), %c(register)
+00100011 aaaabbbb 0000cccc 00000000
+# (31337_37333076617857, `NOR %b and %c and stores the result in %a.`)
+
+add %a(register), %b(register), %c(register)
+00100100 aaaabbbb 0000cccc 00000000
+# (31337_51030225266712, `ADD %b and %c and stores the result in %a.`)
+
+sub %a(register), %b(register), %c(register)
+00100101 aaaabbbb 0000cccc 00000000
+# (31337_28652904157569, `SUB %b and %c and stores the result in %a.`)
+
+xor %a(register), %b(register), %c(register)
+00100110 aaaabbbb 0000cccc 00000000
+# (31337_46967848572964, `XOR %b and %c and stores the result in %a.`)
+
+lsl %a(register), %b(register), %c(register)
+00100111 aaaabbbb 0000cccc 00000000
+# (31337_13576771671264, `Logical shifts left %b by %c and stores the result in %a.`)
+
+lsr %a(register), %b(register), %c(register)
+00101000 aaaabbbb 0000cccc 00000000
+# (31337_67019902294705, `Logical shifts right %b by %c and stores the result in %a.`)
+
+asr %a(register), %b(register), %c(register)
+00101001 aaaabbbb 0000cccc 00000000
+# (31337_42626144645228, `Arithmetic shift right %b by %c and stores the result in %a.`)
+
+cmp %a(register), %b(register)
+00101010 1111aaaa 0000bbbb 00000000
+# (31337_54565380946620, `Compares %a and %b and stores the result in the 'flags' register.`)
+
+
+nand %a(register), %b(register), %c:U16(immediate | label)
+00110000 aaaabbbb cccccccc cccccccc
+# (31337_75302656449127, `NAND %b and %c and stores the result in %a.`)
+
+or %a(register), %b(register), %c:U16(immediate | label)
+00110001 aaaabbbb cccccccc cccccccc
+# (31337_30574433901064, `OR %b and %c and stores the result in %a.`)
+
+nor %a(register), %b(register), %c:U16(immediate | label)
+00110011 aaaabbbb cccccccc cccccccc
+# (31337_46087142860391, `NOR %b and %c and stores the result in %a.`)
+
+and %a(register), %b(register), %c:U16(immediate | label)
+00110010 aaaabbbb cccccccc cccccccc
+# (31337_86713121915282, `AND %b and %c and stores the result in %a.`)
+
+add %a(register), %b(register), %c:U16(immediate | label)
+00110100 aaaabbbb cccccccc cccccccc
+# (31337_23349431010010, `ADD %b and %c and stores the result in %a.`)
+
+sub %a(register), %b(register), %c:U16(immediate | label)
+00110101 aaaabbbb cccccccc cccccccc
+# (31337_67495552371225, `SUB %b and %c and stores the result in %a.`)
+
+xor %a(register), %b(register), %c:U16(immediate | label)
+00110110 aaaabbbb cccccccc cccccccc
+# (31337_66785854415645, `XOR %b and %c and stores the result in %a.`)
+
+lsl %a(register), %b(register), %c:U16(immediate | label)
+00110111 aaaabbbb cccccccc cccccccc
+# (31337_54577249258785, `Logical shifts left %b by %c and stores the result in %a.`)
+
+lsr %a(register), %b(register), %c:U16(immediate | label)
+00111000 aaaabbbb cccccccc cccccccc
+# (31337_16685334571248, `Logical shifts right %b by %c and stores the result in %a.`)
+
+asr %a(register), %b(register), %c:U16(immediate | label)
+00111001 aaaabbbb cccccccc cccccccc
+# (31337_55551510150818, `Arithmetic shifts right %b and %c and stores the result in %a.`)
+
+cmp %a(register), %b:U16(immediate)
+00111010 1111aaaa bbbbbbbb bbbbbbbb
+# (31337_37102964808364, `Compares %a and %b and stores the result in the 'flags' register.`)
+
+
+jmp %a(register)
+01001000 00001111 0000aaaa 00000000
+# (31337_74148826866592, `Jumps to %a.`)
+
+
+jmp %a:U16(immediate | label)
+01011000 00001111 aaaaaaaa aaaaaaaa
+# (31337_90876214388280, `Jumps to %a.`)
+
+je %a:U16(immediate | label)
+01010001 00001111 aaaaaaaa aaaaaaaa
+# (31337_88180305667389, `Jumps to %a if the values were equal (reading results from 'flags').`)
+
+jne %a:U16(immediate | label)
+01011001 00001111 aaaaaaaa aaaaaaaa
+# (31337_67178948469318, `Jumps to %a if the values were not equal (reading results from 'flags').`)
+
+
+jb %a:U16(immediate | label)
+01010010 00001111 aaaaaaaa aaaaaaaa
+# (31337_31763492459833, `Jumps to %a if the first value was below (unsigned) the second (reading results from 'flags').`)
+
+jae %a:U16(immediate | label)
+01011010 00001111 aaaaaaaa aaaaaaaa
+# (31337_82087388048746, `Jumps to %a if the first value was above (unsigned) or equal to the second (reading results from 'flags').`)
+
+jbe %a:U16(immediate | label)
+01010011 00001111 aaaaaaaa aaaaaaaa
+# (31337_26183125927881, `Jumps to %a if the First value was below (unsigned) or equal to the second (reading results from 'flags').`)
+
+ja %a:U16(immediate | label)
+01011011 00001111 aaaaaaaa aaaaaaaa
+# (31337_11626177279992, `Jumps to %a if the first value was above (unsigned) the second (reading results from 'flags').`)
+
+
+jl %a:U16(immediate | label)
+01010100 00001111 aaaaaaaa aaaaaaaa
+# (31337_71637103729391, `Jumps to %a if the first value was less (signed) than the second (reading results from 'flags').`)
+
+jge %a:U16(immediate | label)
+01011100 00001111 aaaaaaaa aaaaaaaa
+# (31337_53499765587663, `Jumps to %a if the first value was greater (signed) or equal to the second (reading results from 'flags').`)
+
+jle %a:U16(immediate | label)
+01010101 00001111 aaaaaaaa aaaaaaaa
+# (31337_77040234358389, `Jumps to %a if the first value was less (signed) or equal to the second (reading results from 'flags').`)
+
+jg %a:U16(immediate | label)
+01011101 00001111 aaaaaaaa aaaaaaaa
+# (31337_71317192057126, `Jumps to %a if the first value was greater (signed) than the second (reading results from 'flags').`)
+
+
+
+load_8 %dest(register), [%adr(register)]
+01100000 dddd0000 0000aaaa 00000000
+# (31337_57200935026773, `Loads address %adr from main memory and stores it to %dest.`)
+
+load_8 %dest(register), [%adr:U16(immediate)]
+01110000 dddd0000 aaaaaaaa aaaaaaaa
+# (31337_57200935026773, `Loads address %adr from main memory and stores it to %dest.`)
+
+load_16 %dest(register), [%adr(register)]
+01100001 dddd0000 0000aaaa 00000000
+# (31337_57200935026773, `Loads address %adr from main memory and stores it to %dest.`)
+
+load_16 %dest(register), [%adr:U16(immediate)]
+01110001 dddd0000 aaaaaaaa aaaaaaaa
+# (31337_57200935026773, `Loads address %adr from main memory and stores it to %dest.`)
+
+load_32 %dest(register), [%adr(register)]
+01100010 dddd0000 0000aaaa 00000000
+# (31337_57200935026773, `Loads address %adr from main memory and stores it to %dest.`)
+
+load_32 %dest(register), [%adr:U16(immediate)]
+01110010 dddd0000 aaaaaaaa aaaaaaaa
+# (31337_57200935026773, `Loads address %adr from main memory and stores it to %dest.`)
+
+pload %dest(register), [%adr(register)]
+01100011 dddd0000 0000aaaa 00000000
+# (31337_89257021109854, `Loads address %adr from persistent memory and stores it to %dest.`)
+
+pload %dest(register), [%adr:U16(immediate)]
+01110011 dddd0000 aaaaaaaa aaaaaaaa
+# (31337_89257021109854, `Loads address %adr from persistent memory and stores it to %dest.`)
+
+store_8 [%adr(register)], %value(register)
+01100100 0000vvvv 0000aaaa 00000000
+# (31337_61040780958703, `Stores %value to address %adr in main memory.`)
+
+store_8 [%adr:U16(immediate)], %value(register)
+01110100 0000vvvv aaaaaaaa aaaaaaaa
+# (31337_61040780958703, `Stores %value to address %adr in main memory.`)
+
+store_16 [%adr(register)], %value(register)
+01100101 0000vvvv 0000aaaa 00000000
+# (31337_61040780958703, `Stores %value to address %adr in main memory.`)
+
+store_16 [%adr:U16(immediate)], %value(register)
+01110101 0000vvvv aaaaaaaa aaaaaaaa
+# (31337_61040780958703, `Stores %value to address %adr in main memory.`)
+
+store_32 [%adr(register)], %value(register)
+01100110 0000vvvv 0000aaaa 00000000
+# (31337_61040780958703, `Stores %value to address %adr in main memory.`)
+
+store_32 [%adr:U16(immediate)], %value(register)
+01110110 0000vvvv aaaaaaaa aaaaaaaa
+# (31337_61040780958703, `Stores %value to address %adr in main memory.`)
+
+pstore [%adr(register)], %value(register)
+01100111 0000vvvv 0000aaaa 00000000
+# (31337_13110303123442, `Stores %value to address %adr in persistent memory.`)
+
+pstore [%adr:U16(immediate)], %value(register)
+01110111 0000vvvv aaaaaaaa aaaaaaaa
+# (31337_13110303123442, `Stores %value to address %adr in persistent memory.`)
+
+
+mov %a(register), %b(register)
+00100001 aaaa0000 0000bbbb 00000000
+# (31337_53941960758392, `Moves a value to %a from %b.`)
+
+mov %a(register), %b:U16(immediate | label)
+00110001 aaaa0000 bbbbbbbb bbbbbbbb
+# (31337_34859363394334, `Moves a value to %a.`)
+
+neg %a(register), %b(register)
+00100101 aaaa0000 0000bbbb 00000000
+# (31337_72894099288447, `Negates a %b and stores it in %a.`)
+
+neg %a(register), %b:U16(immediate | label)
+00110101 aaaa0000 bbbbbbbb bbbbbbbb
+# (31337_57504570243305, `Negates a value and stores it in %a.`)
+
+not %a(register), %b(register)
+00100011 aaaa0000 0000bbbb 00000000
+# (31337_55357258069734, `Toggles the bits of %b and stores it in %a.`)
+
+not %a(register), %b:U16(immediate | label)
+00110011 aaaa0000 bbbbbbbb bbbbbbbb
+# (31337_40508048121543, `Toggles the bits of a value and stores it in %a.`)
+
+
+push %a(register)
+00110101 11101110 00000000 00000100   01100110 0000aaaa 00001110 00000000
+# (31337_28545157055801, `Pushes %a onto the stack. This instruction is a synonym for:   sub sp, sp, 4   store_32 [sp], %a`)
+
+pop %a(register)
+01100010 aaaa0000 00001110 00000000   00110100 11101110 00000000 00000100
+# (31337_54469754973269, `Pops %a off the stack. This instruction is a synonym for:   load_32 %a, [sp]   add sp, sp, 4`)
+
+call %a(label)
+00000111 11110000 00000000 00000000   00110100 11111111 00000000 00010100   00110101 11101110 00000000 00000100   01100110 00001111 00001110 00000000   01011000 00001111 aaaaaaaa aaaaaaaa
+# (31337_57788320241950, `Calls %a. Overwrites the flag register. This instruction is a synonym for:   counter flags   add flags, flags, 20   sub sp, sp, 4   store_32 [sp], flags   jmp %a`)
+
+ret
+01100010 11110000 00001110 00000000   00110100 11101110 00000000 00000100   01001000 00001111 00001111 00000000
+# (31337_48332460366102, `Returns from the last function call. Overwrites the flag register. This instruction is a synonym for:   load_32 flags, [sp]   add sp, sp, 4   jmp flags`)
+
diff --git a/isa/test.isa b/isa/test.isa
new file mode 100644
index 0000000..45b78ec
--- /dev/null
+++ b/isa/test.isa
@@ -0,0 +1,361 @@
+[settings]
+name = "Symphony"
+
+[fields]
+
+register
+zr 0000
+r1 0001
+r2 0010
+r3 0011
+r4 0100
+r5 0101
+r6 0110
+r7 0111
+r8 1000
+r9 1001
+r10 1010
+r11 1011
+r12 1100
+r13 1101
+sp 1110
+flags 1111
+
+condition
+""     1000
+".all" 1000
+".e"   0001
+".ne"  1001
+".b"   0010
+".ae"  1010
+".be"  0011
+".a"   1011
+".l"   0100
+".ge"  1100
+".le"  0101
+".g"   1101
+
+[instructions]
+
+nop
+00000000 00000000 00000000 00000000
+# (31337_88272577071256, `Does nothing.`)
+
+in %a(register)
+00000001 aaaa0000 00000000 00000000
+# (31337_44524771082050, `Loads an input and stores it in %a.`)
+
+out %b(register)
+00000010 00000000 0000bbbb 00000000
+# (31337_18254089726214, `Sends %b to output.`)
+
+out %a:U16(immediate)
+00010010 00000000 aaaaaaaa aaaaaaaa
+# (31337_15915531259935, `Sends %a to output.`)
+
+keyboard %a(register)
+00000011 aaaa0000 00000000 00000000
+# (31337_27868418224252, `Loads keyboard input and stores it to %a.`)
+
+screen %a(register), %b(register)
+00000100 0000aaaa 0000bbbb 00000000
+# (31337_39857449355186, `Stores the value of %b in screen setting %a.`)
+
+screen %a(register), %b:U16(immediate)
+00010100 0000aaaa bbbbbbbb bbbbbbbb
+# (31337_12835125621741, `Stores the value of %b in screen setting %a.`)
+
+time_0 %a(register)
+00000101 aaaa0000 00000000 00000000
+# (31337_72551400437977, `Loads the lower four bytes of the time value and stores it to %a.`)
+
+time_1 %a(register)
+00000110 aaaa0000 00000000 00000000
+# (31337_68219187873304, `Loads the upper four bytes of the time value and stores it to %a.`)
+
+counter %a(register)
+00000111 aaaa0000 00000000 00000000
+# (31337_67376584796076, `Loads the counter value and stores it to %a.`)
+
+
+
+nand %a(register), %b(register), %c(register)
+00100000 aaaabbbb 0000cccc 00000000
+# (31337_86705346481666, `NAND %b and %c and stores the result in %a.`)
+
+or %a(register), %b(register), %c(register)
+00100001 aaaabbbb 0000cccc 00000000
+# (31337_40732211165530, `OR %b and %c and stores the result in %a.`)
+
+and %a(register), %b(register), %c(register)
+00100010 aaaabbbb 0000cccc 00000000
+# (31337_85679146831122, `AND %b and %c and stores the result in %a.`)
+
+nor %a(register), %b(register), %c(register)
+00100011 aaaabbbb 0000cccc 00000000
+# (31337_37333076617857, `NOR %b and %c and stores the result in %a.`)
+
+add %a(register), %b(register), %c(register)
+00100100 aaaabbbb 0000cccc 00000000
+# (31337_51030225266712, `ADD %b and %c and stores the result in %a.`)
+
+sub %a(register), %b(register), %c(register)
+00100101 aaaabbbb 0000cccc 00000000
+# (31337_28652904157569, `SUB %b and %c and stores the result in %a.`)
+
+xor %a(register), %b(register), %c(register)
+00100110 aaaabbbb 0000cccc 00000000
+# (31337_46967848572964, `XOR %b and %c and stores the result in %a.`)
+
+lsl %a(register), %b(register), %c(register)
+00100111 aaaabbbb 0000cccc 00000000
+# (31337_13576771671264, `Logical shifts left %b by %c and stores the result in %a.`)
+
+lsr %a(register), %b(register), %c(register)
+00101000 aaaabbbb 0000cccc 00000000
+# (31337_67019902294705, `Logical shifts right %b by %c and stores the result in %a.`)
+
+asr %a(register), %b(register), %c(register)
+00101001 aaaabbbb 0000cccc 00000000
+# (31337_42626144645228, `Arithmetic shift right %b by %c and stores the result in %a.`)
+
+cmp %a(register), %b(register)
+00101010 1111aaaa 0000bbbb 00000000
+# (31337_54565380946620, `Compares %a and %b and stores the result in the 'flags' register.`)
+
+
+nand %a(register), %b(register), %c:U16(immediate | label)
+00110000 aaaabbbb cccccccc cccccccc
+# (31337_75302656449127, `NAND %b and %c and stores the result in %a.`)
+
+or %a(register), %b(register), %c:U16(immediate | label)
+00110001 aaaabbbb cccccccc cccccccc
+# (31337_30574433901064, `OR %b and %c and stores the result in %a.`)
+
+nor %a(register), %b(register), %c:U16(immediate | label)
+00110011 aaaabbbb cccccccc cccccccc
+# (31337_46087142860391, `NOR %b and %c and stores the result in %a.`)
+
+and %a(register), %b(register), %c:U16(immediate | label)
+00110010 aaaabbbb cccccccc cccccccc
+# (31337_86713121915282, `AND %b and %c and stores the result in %a.`)
+
+add %a(register), %b(register), %c:U16(immediate | label)
+00110100 aaaabbbb cccccccc cccccccc
+# (31337_23349431010010, `ADD %b and %c and stores the result in %a.`)
+
+sub %a(register), %b(register), %c:U16(immediate | label)
+00110101 aaaabbbb cccccccc cccccccc
+# (31337_67495552371225, `SUB %b and %c and stores the result in %a.`)
+
+xor %a(register), %b(register), %c:U16(immediate | label)
+00110110 aaaabbbb cccccccc cccccccc
+# (31337_66785854415645, `XOR %b and %c and stores the result in %a.`)
+
+lsl %a(register), %b(register), %c:U16(immediate | label)
+00110111 aaaabbbb cccccccc cccccccc
+# (31337_54577249258785, `Logical shifts left %b by %c and stores the result in %a.`)
+
+lsr %a(register), %b(register), %c:U16(immediate | label)
+00111000 aaaabbbb cccccccc cccccccc
+# (31337_16685334571248, `Logical shifts right %b by %c and stores the result in %a.`)
+
+asr %a(register), %b(register), %c:U16(immediate | label)
+00111001 aaaabbbb cccccccc cccccccc
+# (31337_55551510150818, `Arithmetic shifts right %b and %c and stores the result in %a.`)
+
+cmp %a(register), %b:U16(immediate | label)
+00111010 1111aaaa bbbbbbbb bbbbbbbb
+# (31337_37102964808364, `Compares %a and %b and stores the result in the 'flags' register.`)
+
+
+jmp %a(register)
+01001000 00001111 0000aaaa 00000000
+# (31337_74148826866592, `Jumps to %a.`)
+
+
+jmp %a:U16(immediate | label)
+01011000 00001111 aaaaaaaa aaaaaaaa
+# (31337_90876214388280, `Jumps to %a.`)
+
+je %a:U16(immediate | label)
+01010001 00001111 aaaaaaaa aaaaaaaa
+# (31337_88180305667389, `Jumps to %a if the values were equal (reading results from 'flags').`)
+
+jne %a:U16(immediate | label)
+01011001 00001111 aaaaaaaa aaaaaaaa
+# (31337_67178948469318, `Jumps to %a if the values were not equal (reading results from 'flags').`)
+
+
+jb %a:U16(immediate | label)
+01010010 00001111 aaaaaaaa aaaaaaaa
+# (31337_31763492459833, `Jumps to %a if the first value was below (unsigned) the second (reading results from 'flags').`)
+
+jae %a:U16(immediate | label)
+01011010 00001111 aaaaaaaa aaaaaaaa
+# (31337_82087388048746, `Jumps to %a if the first value was above (unsigned) or equal to the second (reading results from 'flags').`)
+
+jbe %a:U16(immediate | label)
+01010011 00001111 aaaaaaaa aaaaaaaa
+# (31337_26183125927881, `Jumps to %a if the First value was below (unsigned) or equal to the second (reading results from 'flags').`)
+
+ja %a:U16(immediate | label)
+01011011 00001111 aaaaaaaa aaaaaaaa
+# (31337_11626177279992, `Jumps to %a if the first value was above (unsigned) the second (reading results from 'flags').`)
+
+
+jl %a:U16(immediate | label)
+01010100 00001111 aaaaaaaa aaaaaaaa
+# (31337_71637103729391, `Jumps to %a if the first value was less (signed) than the second (reading results from 'flags').`)
+
+jge %a:U16(immediate | label)
+01011100 00001111 aaaaaaaa aaaaaaaa
+# (31337_53499765587663, `Jumps to %a if the first value was greater (signed) or equal to the second (reading results from 'flags').`)
+
+jle %a:U16(immediate | label)
+01010101 00001111 aaaaaaaa aaaaaaaa
+# (31337_77040234358389, `Jumps to %a if the first value was less (signed) or equal to the second (reading results from 'flags').`)
+
+jg %a:U16(immediate | label)
+01011101 00001111 aaaaaaaa aaaaaaaa
+# (31337_71317192057126, `Jumps to %a if the first value was greater (signed) than the second (reading results from 'flags').`)
+
+
+
+load_8 %dest(register), [%adr(register)]
+01100000 dddd0000 0000aaaa 00000000
+# (31337_57200935026773, `Loads address %adr from main memory and stores it to %dest.`)
+
+load_8 %dest(register), [%adr:U16(immediate | label)]
+01110000 dddd0000 aaaaaaaa aaaaaaaa
+# (31337_57200935026773, `Loads address %adr from main memory and stores it to %dest.`)
+
+load_16 %dest(register), [%adr(register)]
+01100001 dddd0000 0000aaaa 00000000
+# (31337_57200935026773, `Loads address %adr from main memory and stores it to %dest.`)
+
+load_16 %dest(register), [%adr:U16(immediate | label)]
+01110001 dddd0000 aaaaaaaa aaaaaaaa
+# (31337_57200935026773, `Loads address %adr from main memory and stores it to %dest.`)
+
+load_32 %dest(register), [%adr(register)]
+01100010 dddd0000 0000aaaa 00000000
+# (31337_57200935026773, `Loads address %adr from main memory and stores it to %dest.`)
+
+load_32 %dest(register), [%adr:U16(immediate | label)]
+01110010 dddd0000 aaaaaaaa aaaaaaaa
+# (31337_57200935026773, `Loads address %adr from main memory and stores it to %dest.`)
+
+pload %dest(register), [%adr(register)]
+01100011 dddd0000 0000aaaa 00000000
+# (31337_89257021109854, `Loads address %adr from persistent memory and stores it to %dest.`)
+
+pload %dest(register), [%adr:U16(immediate)]
+01110011 dddd0000 aaaaaaaa aaaaaaaa
+# (31337_89257021109854, `Loads address %adr from persistent memory and stores it to %dest.`)
+
+store_8 [%adr(register)], %value(register)
+01100100 0000vvvv 0000aaaa 00000000
+# (31337_61040780958703, `Stores %value to address %adr in main memory.`)
+
+store_8 [%adr:U16(immediate | label)], %value(register)
+01110100 0000vvvv aaaaaaaa aaaaaaaa
+# (31337_61040780958703, `Stores %value to address %adr in main memory.`)
+
+store_16 [%adr(register)], %value(register)
+01100101 0000vvvv 0000aaaa 00000000
+# (31337_61040780958703, `Stores %value to address %adr in main memory.`)
+
+store_16 [%adr:U16(immediate | label)], %value(register)
+01110101 0000vvvv aaaaaaaa aaaaaaaa
+# (31337_61040780958703, `Stores %value to address %adr in main memory.`)
+
+store_32 [%adr(register)], %value(register)
+01100110 0000vvvv 0000aaaa 00000000
+# (31337_61040780958703, `Stores %value to address %adr in main memory.`)
+
+store_32 [%adr:U16(immediate | label)], %value(register)
+01110110 0000vvvv aaaaaaaa aaaaaaaa
+# (31337_61040780958703, `Stores %value to address %adr in main memory.`)
+
+pstore [%adr(register)], %value(register)
+01100111 0000vvvv 0000aaaa 00000000
+# (31337_13110303123442, `Stores %value to address %adr in persistent memory.`)
+
+pstore [%adr:U16(immediate)], %value(register)
+01110111 0000vvvv aaaaaaaa aaaaaaaa
+# (31337_13110303123442, `Stores %value to address %adr in persistent memory.`)
+
+
+mov %a(register), %b(register)
+00100001 aaaa0000 0000bbbb 00000000
+# (31337_53941960758392, `Moves a value to %a from %b.`)
+
+mov %a(register), %b:U16(immediate | label)
+00110001 aaaa0000 bbbbbbbb bbbbbbbb
+# (31337_34859363394334, `Moves a value to %a.`)
+
+neg %a(register), %b(register)
+00100101 aaaa0000 0000bbbb 00000000
+# (31337_72894099288447, `Negates a %b and stores it in %a.`)
+
+neg %a(register), %b:U16(immediate | label)
+00110101 aaaa0000 bbbbbbbb bbbbbbbb
+# (31337_57504570243305, `Negates a value and stores it in %a.`)
+
+not %a(register), %b(register)
+00100011 aaaa0000 0000bbbb 00000000
+# (31337_55357258069734, `Toggles the bits of %b and stores it in %a.`)
+
+not %a(register), %b:U16(immediate | label)
+00110011 aaaa0000 bbbbbbbb bbbbbbbb
+# (31337_40508048121543, `Toggles the bits of a value and stores it in %a.`)
+
+
+push %a(register)
+00110101 11101110 00000000 00000100   01100110 0000aaaa 00001110 00000000
+# (31337_28545157055801, `Pushes %a onto the stack. This instruction is a synonym for:   sub sp, sp, 4   store_32 [sp], %a`)
+
+pop %a(register)
+01100010 aaaa0000 00001110 00000000   00110100 11101110 00000000 00000100
+# (31337_54469754973269, `Pops %a off the stack. This instruction is a synonym for:   load_32 %a, [sp]   add sp, sp, 4`)
+
+call %a(label)
+00000111 11110000 00000000 00000000   00110100 11111111 00000000 00010100   00110101 11101110 00000000 00000100   01100110 00001111 00001110 00000000   01011000 00001111 aaaaaaaa aaaaaaaa
+# (31337_57788320241950, `Calls %a. Overwrites the flag register. This instruction is a synonym for:   counter flags   add flags, flags, 20   sub sp, sp, 4   store_32 [sp], flags   jmp %a`)
+
+ret
+01100010 11110000 00001110 00000000   00110100 11101110 00000000 00000100   01001000 00001111 00001111 00000000
+# (31337_48332460366102, `Returns from the last function call. Overwrites the flag register. This instruction is a synonym for:   load_32 flags, [sp]   add sp, sp, 4   jmp flags`)
+
+
+li %r(register), %a:S33(immediate | label)
+; assert(0 - 0x80000000 <= %a, "%a is too low")
+; assert(%a <= 0xffffffff, "%a is too high")
+%s = trailing_zeros(%a)
+%sa = %a >> %s
+%nota = %a ^ 0xffffffff
+when %a <=u 0xffff   : 00110001 %r[3:0]0000 %a[15:0]                                                 ; First 64k
+when %nota <=u 0xffff: 00110001 %r[3:0]0000 %nota[15:0]    00100011 %r[3:0]0000 0000%r[3:0] 00000000 ; Last 64k
+when %sa <=u 0xffff  : 00110001 %r[3:0]0000 %sa[15:0]      00110111 %r[3:0]%r[3:0] 00000000 %s[7:0]  ; Aligned
+00110001 %r[3:0]0000 %a[31:16]   00110111 %r[3:0]%r[3:0] 00000000 00010000   00110001 %r[3:0]%r[3:0] %a[15:0]          ; Any 32-bit
+# Load 32-bit immediate %a into %r
+
+qcall %a:U32(label)
+; assert(0 - 0x80000000 <= %a, "%a is too low")
+; assert(%a <= 0xffffffff, "%a is too high")
+%r = 13
+%f = 15
+%s = trailing_zeros(%a)
+%sa = %a >> %s
+%nota = %a ^ 0xffffffff
+when %a <=u 0xffff   : 01011000 %r[3:0]ffff %a[15:0]                                                                                              ; First 64k
+when %nota <=u 0xffff: 00110001 %r[3:0]0000 %nota[15:0]    00100011 %r[3:0]0000 0000%r[3:0] 00000000    01001000 %r[3:0]ffff 0000%r[3:0] 00000000 ; Last 64k
+when %sa <=u 0xffff  : 00110001 %r[3:0]0000 %sa[15:0]      00110111 %r[3:0]%r[3:0] 00000000 %s[7:0]     01001000 %r[3:0]ffff 0000%r[3:0] 00000000 ; Aligned
+00110001 %r[3:0]0000 %a[31:16]   00110111 %r[3:0]%r[3:0] 00000000 00010000   00110001 %r[3:0]%r[3:0] %a[15:0]    01001000 %r[3:0]ffff 0000%r[3:0] 00000000 ; Any 32-bit
+# Jump and link to 32-bit label
+
+halt%c(condition)
+1000cccc 00001111 00000000 00000000
+# Halt on condition %c
+
diff --git a/src/errno.asm b/src/errno.asm
new file mode 100755
index 0000000..5d06c89
--- /dev/null
+++ b/src/errno.asm
@@ -0,0 +1,27 @@
+; Error numbers should be 16 bit ODD numbers, so they can be loaded as immediates
+; and can be checked with:
+;
+;   qcall falible_function
+;   jne falible_ok ; Jump no error
+;       ; handle error
+;   falible_ok:
+;   ; Happy path
+;   ; ...
+; OR
+;   qcall falible_function
+;   je handle_error ; Jump if error
+;   ; Happy path
+;   ; ...
+;   handle_error:
+;       ; handle error
+
+pub const OK                   = 0x0000
+pub const SCREEN_INVALID_MODE  = 0x0001
+pub const SCREEN_INVALID_WIDTH = 0x0003
+pub const SCREEN_FB_TOO_SMALL  = 0x0005
+pub const SCREEN_OUTSIDE_FB    = 0x0007
+
+pub const MAGIC_BAD            = 0x8001
+
+; Extended error codes, these would require loading a 32 bit value.
+pub const EXT_OK              = 0x00000000
\ No newline at end of file
diff --git a/src/globals.asm b/src/globals.asm
new file mode 100755
index 0000000..d9398c5
--- /dev/null
+++ b/src/globals.asm
@@ -0,0 +1,30 @@
+; Addresses of global variables
+
+pub const MAGIC_ADDRESS_LONG = 0x0c       ; U32 Location of the magic value
+pub const MAGIC_VALUE_LONG   = 0xb301534c ; U32 Full 32 bits of the magic value
+pub const MAGIC_ADDRESS      = 0x0e       ; U16 Location of the low 16 bits of magic value
+pub const MAGIC_VALUE        = 0x534c     ; U16 Low 16 bits of the magic value
+
+; If not double buffering these point to the same buffer
+; If double buffering they must the same size
+pub const FB_DISPLAY_PTR     = 0x10       ; U32 Currently displayed buffer
+pub const FB_DISPLAY_STRIDE  = 0x14       ; U16 Bytes in each row of the displayed buffer
+pub const FB_DISPLAY_DEPTH   = 0x16       ; U16 Bits per pixel of the display buffer
+pub const FB_DRAW_PTR        = 0x18       ; U32 Draw to this buffer
+pub const FB_DRAW_STRIDE     = 0x1c       ; U16 Bytes in each row of the draw buffer == FB_DISPLAY_STRIDE
+pub const FB_DRAW_DEPTH      = 0x1e       ; U16 Bits per pixel of the draw buffer == FB_DISPLAY_DEPTH
+pub const FB_SIZE_BYTE       = 0x20       ; U32 In bytes
+pub const FB_WIDTH_PX        = 0x24       ; U16 Width of screen in pixels
+pub const FB_HEIGHT_PX       = 0x26       ; U16 Height of screen in pixels
+
+; Log2 of width in pixels, e.g.
+; * 10 => 1024 * 768
+; *  8 =>  256 * 192
+pub const FB_LOG_WIDTH       = 0x28       ; U8
+pub const FB_LOG_STRIDE      = 0x29       ; U8 Log2 of FB_xxx_STRIDE in bytes
+
+pub const FB_BYTES_PER_PIXEL = 0x2a       ; U8 Specialisations should hardcode this
+; Log2 of bytes per pixel
+; * 0 => 8 bits per pixel
+; * 2 => 32 bits per pixel
+pub const FB_LOG_BPP         = 0x2b       ; U8 Specialisations should hardcode this
\ No newline at end of file
diff --git a/src/imath.asm b/src/imath.asm
index 5d8fbd2..a246a2c 100644
--- a/src/imath.asm
+++ b/src/imath.asm
@@ -4,29 +4,51 @@
 ; r2 - The second value
 ; Result:
 ; r1 - The lower 32 bits of the result
-; Clobbers: r2, r3, r4, r5
+; Clobbers: r2, r3, r4, r5, r6
 pub mul_low:
-	add r3, r2, r2 ; r3 = r2 * 2
+	cmp r1, r2
+	jbe mul_low_noswap
+	xor r1, r2, r1
+	xor r2, r1, r2
+	xor r1, r2, r1
+	; Passthrough
+
+; Multiplies r1 and r2, returning the lower part of the result
+; This method assumes r1 is smaller than r2, which results in faster execution
+; Arguments:
+; r1 - The first value
+; r2 - The second value
+; Result:
+; r1 - The lower 32 bits of the result
+; Clobbers: r2, r3, r4, r5, r6
+pub mul_low_noswap:
 	mov r4, 0 ; r4 has the result
-
-	mul_low_loop:
-
-		and r5, r1, 1 ; a0
-		neg r5, r5 ; mask
-		and r5, r2, r5 ; a0 ? r2 : 0
-		add r4, r4, r5
-
-		and r5, r1, 2 ; a1 is now 0 or 2
-		lsr r5, r5, 1 ; normalize to 0/1
-		neg r5, r5 ; mask
-		and r5, r3, r5 ; a1 ? r2 * 2 : 0
-		add r4, r4, r5
-
-		lsl r2, r2, 2
-		lsl r3, r3, 2
-		lsr r1, r1, 2
-		cmp r1, zr
-		jne mul_low_loop
+	mov r6, mul_loop_end
+	add r3, r2, r2
+	
+	mul_loop:
+		and r5, r1, 14 ; Taking the first four bits, discarding the odd
+		lsl r5, r5, 1 ; 0000 - 0, 0001 - 2, 0010 - 4, 0011 - 8, etc.
+		sub r5, r6, r5
+		jmp r5
+		
+		add r4, r4, r3
+		add r4, r4, r3
+		add r4, r4, r3
+		add r4, r4, r3
+		add r4, r4, r3
+		add r4, r4, r3
+		add r4, r4, r3
+		mul_loop_end:
+			mov flags, r1
+			jne mul_skip_one
+			add r4, r4, r2
+			mul_skip_one:
+			lsl r2, r2, 4
+			lsl r3, r3, 4
+			lsr r1, r1, 4
+			cmp r1, zr
+			jne mul_loop
 
 	mov r1, r4
 	jmp r13
diff --git a/src/start.asm b/src/start.asm
new file mode 100755
index 0000000..c7e108f
--- /dev/null
+++ b/src/start.asm
@@ -0,0 +1,19 @@
+;@0 ; Reserve space for globals
+
+; First initialise the stack pointer
+nor sp, zr, 0xffff
+; Jump over the globals to user code
+jmp 0x100
+
+U32 0 ; 4 bytes
+
+; MAGIC_VALUE
+;@0x0c
+U32 0xb301534c
+
+; Pad with zeroes since the @addr feature is currently broken.
+; * https://discord.com/channels/828292123936948244/1545010596246847568
+;
+U1920 0  ; 240 bytes
+
+;@0x100 ; start of user code
\ No newline at end of file
diff --git a/src/stdlib.asm b/src/stdlib.asm
index 512c6c3..ee6477d 100644
--- a/src/stdlib.asm
+++ b/src/stdlib.asm
@@ -3,6 +3,7 @@ pub include imath
 pub include array
 pub include console
 pub include mem
+pub include string_to_int
 
 ; Needs to be last!
 pub include LUTs
diff --git a/src/string_to_int.asm b/src/string_to_int.asm
new file mode 100644
index 0000000..cd961c2
--- /dev/null
+++ b/src/string_to_int.asm
@@ -0,0 +1,188 @@
+; Internal register assignments:
+; r1 - Partially parsed output integer
+; r2 - Parsing position
+; r3 - Character being parsed
+; r4 - Set to -1 if the input is negative
+
+; Convert string to integer (akin to libc atoi(), strotoi())
+; Arguments:
+; r1 - Pointer to string
+; Result:
+; r1 - Parsed integer
+; r2 - Pointer to first rejected input byte
+; Clobbers:
+; flags
+; r3 - last character read
+; r4 - Negative marker
+; Note: Unless the input is "0", tail-calls into a base-specific specialization.
+pub auto:
+mov r2, r1
+mov r1, 0
+mov r4, 0
+load_8 r3, [r2]
+cmp r3, 0x2D ; '-'
+jne auto_positive
+sub r4, zr, 1           ; Set r4 to -1
+add r2, r2, 1
+auto_positive:
+load_16 r3, [r2]        ; 2-byte prefix "0b", "0o", "0x", etc.
+or r3, r3, 0x20         ; 2nd char to lower case
+add r2, r2, 2
+cmp r3, 0x3062 ; "0b"
+je bin_loop
+cmp r3, 0x306F ; "0o"
+je oct_loop
+cmp r3, 0x3078 ; "0x"
+je hex_loop
+sub r2, r2, 2           ; no matching prefix, move pointer back
+jmp dec_loop
+
+; Epilogue
+done:
+add r1, r1, r4          ; If r4 is -1, negate r1. Else it's 0 and no effect.
+xor r1, r1, r4
+mov flags, 0            ; No error
+jmp r13
+
+; Convert decimal string to integer
+; Arguments:
+; r1 - Pointer to string
+; Result:
+; r1 - Parsed integer
+; r2 - Pointer to first rejected input byte
+; Clobbers:
+; flags
+; r3 - last character read
+; r4 - Negative marker
+pub dec:
+mov r2, r1
+mov r1, 0
+mov r4, 0
+load_8 r3, [r2]
+cmp r3, 0x2D ; '-'
+jne dec_positive
+sub r4, zr, 1           ; Set r4 to -1
+add r2, r2, 1
+dec_loop:
+load_8 r3, [r2]
+dec_positive:
+sub r3, r3, 0x30 ; '0'
+cmp r3, 9
+ja done
+add r2, r2, 1
+lsl flags, r1, 2        ; Use flags to help multiply by 10
+add r1, r1, flags
+lsl r1, r1, 1
+add r1, r1, r3
+jmp dec_loop
+
+; Convert binary string to integer
+; Arguments:
+; r1 - Pointer to string
+; Result:
+; r1 - Parsed integer
+; r2 - Pointer to first rejected input byte
+; Clobbers:
+; flags
+; r3 - last character read
+; r4 - Negative marker
+pub bin:
+mov r2, r1
+mov r1, 0
+mov r4, 0
+load_8 r3, [r2]
+cmp r3, 0x2D ; '-'
+jne bin_positive
+sub r4, zr, 1           ; Set r4 to -1
+add r2, r2, 1
+bin_positive:
+load_16 r3, [r2]        ; check for prefix
+or r3, r3, 0x20         ; 2nd char to lower case
+cmp r3, 0x3062 ; "0b"
+jne bin_loop
+add r2, r2, 2
+bin_loop:
+load_8 r3, [r2]
+sub r3, r3, 0x30 ; '0'
+cmp r3, 1
+ja done
+add r2, r2, 1
+lsl r1, r1, 1
+add r1, r1, r3
+jmp bin_loop
+
+; Convert octal string to integer
+; Arguments:
+; r1 - Pointer to string
+; Result:
+; r1 - Parsed integer
+; r2 - Pointer to first rejected input byte
+; Clobbers:
+; flags
+; r3 - last character read
+; r4 - Negative marker
+pub oct:
+mov r2, r1
+mov r1, 0
+mov r4, 0
+load_8 r3, [r2]
+cmp r3, 0x2D ; '-'
+jne oct_positive
+sub r4, zr, 1           ; Set r4 to -1
+add r2, r2, 1
+oct_positive:
+load_16 r3, [r2]        ; check for prefix
+or r3, r3, 0x20         ; 2nd char to lower case
+cmp r3, 0x306F ; "0o"
+jne oct_loop
+add r2, r2, 2
+oct_loop:
+load_8 r3, [r2]
+sub r3, r3, 0x30 ; '0'
+cmp r3, 7
+ja done
+add r2, r2, 1
+lsl r1, r1, 3
+add r1, r1, r3
+jmp oct_loop
+
+; Convert hexadecimal string to integer
+; Arguments:
+; r1 - Pointer to string
+; Result:
+; r1 - Parsed integer
+; r2 - Pointer to first rejected input byte
+; Clobbers:
+; flags
+; r3 - last character read
+; r4 - Negative marker
+pub hex:
+mov r2, r1
+mov r1, 0
+mov r4, 0
+load_8 r3, [r2]
+cmp r3, 0x2D ; '-'
+jne hex_positive
+sub r4, zr, 1           ; Set r4 to -1
+add r2, r2, 1
+hex_positive:
+load_16 r3, [r2]        ; check for prefix
+or r3, r3, 0x20         ; 2nd char to lower case
+cmp r3, 0x3078 ; "0x"
+jne hex_loop
+add r2, r2, 2
+hex_loop:
+load_8 r3, [r2]
+sub r3, r3, 0x30 ; '0'
+cmp r3, 10
+jb hex_add
+sub r3, r3, 0x11 ; 'A' - '0'
+and r3, r3, 0xdf        ; to lower case
+cmp r3, 5               ; 0-5: 6 letters
+ja done
+add r3, r3, 10          ; Adjust for digits below
+hex_add:
+add r2, r2, 1
+lsl r1, r1, 4
+add r1, r1, r3
+jmp hex_loop
diff --git a/tests/string_to_int_test.asm b/tests/string_to_int_test.asm
new file mode 100644
index 0000000..c60494b
--- /dev/null
+++ b/tests/string_to_int_test.asm
@@ -0,0 +1,57 @@
+; test harness for string_to_int
+; Registers:
+; r8 - test address
+; r9 - expected result
+; r10 - actual length
+; r11 - expected length
+li r8, tests
+li r12, end_of_tests
+next_test:
+; Set up arguments and run test
+add r1, r8, 5          ; Start of test string
+mov r13, atoi_test_ret
+jmp stdlib.string_to_int.auto
+atoi_test_ret:
+
+; Load reference data
+load_32 r9, [r8]       ; Expected result
+sub r10, r2, r8
+sub r10, r10, 5        ; Actual string length
+add r11, r8, 4         ; Address of expected length
+load_8 r11, [r11]      ; Expected length
+
+; Verify results
+cmp r1, r9
+incorrect_result: jne incorrect_result
+cmp r10, r11
+incorrect_length: jne incorrect_length
+
+; Next test
+add r8, r8, 0x10
+cmp r8, r12
+jl next_test
+
+; Done
+success: jmp success
+
+include ../src/stdlib
+
+@0x20000
+tests:
+; inlen is the number of input bytes the function is expected to consume
+; addr  result          inlen  instr
+@0x20000 U32 0           U8 0   "\0"
+@0x20010 U32 0           U8 1   "0\0"
+@0x20020 U32 1           U8 1   "1\0"
+@0x20030 U32 2           U8 1   "2:\0"
+@0x20040 U32 42          U8 2   "42\0"
+@0x20050 U32 67          U8 2   "67lol\0"
+@0x20060 U32 0x69a       U8 5   "0x69a@\0"
+@0x20070 U32 0x4B4       U8 5   "0x4B4g\0"
+@0x20080 U32 0o23        U8 4   "0o239\0"
+@0x20090 U32 0b1011      U8 6   "0b1011\0"
+@0x200a0 U32 0b10001     U8 7   "0b100012\0"
+@0x200b0 U32 0xFFFFFFEB  U8 3   "-21\0"       ; Yuk
+@0x200c0
+end_of_tests:
+