``` 30 a+=1: GO TO 20 40 DATA 1,99,0,201 ``` This will stop with the report **E Out of DATA** when it has filled in the four bytes you specified. ### Using USR to run machine code To run the machine code, you use the function **USR** or its –preferred– **BANK** command variant. In its simplest form **USR** must be provided with a numeric argument, i.e. the starting address or the bank offset. Its result is the value of the **BC** register on return from the machine code program, so assuming you type: ``` PRINT USR 65268 ``` It will return the value **99**. The return address to *NextBASIC* is stacked in the usual way, so return is by a Z80 **ret** instruction. You should not use the **IY** and **I** registers in a machine code routine that expects to use the *NextBASIC* interrupt mechanism. To perform the exact same function by using the **BANK** variant, make the following changes to our program: ``` 10 %a=0 20 BANK NEW %b 30 READ %n : BANK %b POKE %a,%n 40 %a+=1: GO TO 30 50 DATA 1,99,0,201 ``` **RUN** it and you'll see the **E Out of Data** error again; Now it's time to execute it and it's done by giving: ``` PRINT % BANK b USR 0 ``` There are a few more variants of **USR** that differ in key points and make the life of the machine-code programmer a bit easier. These are: **USR$** *addr*\ **BANK** *n* **USR$** *offset* which call the machine code routine at *addr* (or *offset* in bank *n*). Instead however of returning the 16-bit number found in **BC** (as with **USR** *addr*), **USR$** returns a string, defined by the start address returned by the machine-code routine in **DE** and length in **BC**. Additionally, **USR** as well as **USR$** (and their **BANK** variants) can be provided with optional parameters like: **USR**(*addr, param1*[, *param2* [, *param3*...]]]) **USR$**(*addr, param1*[, *param2* [, *param3*...]]]) **BANK** *n* **USR**(*addr, param1*[, *param2* [, *param3*...]]]) **BANK** *n* **USR$**(*addr, param1*[, *param2* [, *param3*...]]]) which can be passed to the machine-code routine (instead of just the start address in **BC**). If a single additional parameter (*param1*) is present, this is passed in **BC** (if it is numeric) or as an address **DE** and length **BC** (if it is a string). The type of the parameter passed to the routine is indicated by the *zero flag*: if set, the parameter is a string (in **DE**,**BC**); if clear, the parameter is a number in **BC**.