# Atmega8A BOOTSZ fuses smaller than urboot size ? (using minicore) **URL:** \ **Category:** Uploading\ **Created:** [September 23, 2025, 10:11am UTC](https://forum.arduino.cc/t/atmega8a-bootsz-fuses-smaller-than-urboot-size-using-minicore/1407276 "2025-09-23T10:11:15Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![nipil](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/nipil/32/1128570_2.png) [@nipil](https://forum.arduino.cc/u/nipil)\ **Post date:** [September 23, 2025, 10:11am UTC](https://forum.arduino.cc/t/atmega8a-bootsz-fuses-smaller-than-urboot-size-using-minicore/1407276/1 "2025-09-23T10:11:15Z")
Hello I burnt a bootloader (and fuses) via Minicore on Atmega8A, using an "Arduino as ISP" UNO. As shown below, it was successful, …. but on second look, something looks weird 😮 ```arduino "C:\Users\xxxxx\AppData\Local\Arduino15\packages\MiniCore\tools\avrdude\8.0-arduino.1/bin/avrdude" "-CC:\Users\xxxxx\AppData\Local\Arduino15\packages\MiniCore\tools\avrdude\8.0-arduino.1/etc/avrdude.conf" -v -patmega8 -cstk500v1 -PCOM5 -b19200 -e -Ulock:w:0xff:m -Uefuse:w:{bootloader.extended_fuses}:m -Uhfuse:w:0b11010111:m -Ulfuse:w:0b00100100:m Avrdude version 8.0-arduino.1 Copyright see https://github.com/avrdudes/avrdude/blob/main/AUTHORS System wide configuration file is C:\Users\xxxxx\AppData\Local\Arduino15\packages\MiniCore\tools\avrdude\8.0-arduino.1\etc\avrdude.conf Using port : COM5 Using programmer : stk500v1 Setting baud rate : 19200 AVR part : ATmega8 Programming modes : SPM, ISP, HVPP Programmer type : STK500 Description : Atmel STK500 v1 HW Version : 2 FW Version : 1.18 Topcard : Unknown Vtarget : 0.0 V Varef : 0.0 V Oscillator : Off SCK period : 0.0 us XTAL frequency : 7.372800 MHz AVR device initialized and ready to accept instructions Device signature = 1E 93 07 (ATmega8, ATmega8A) Erased chip Processing -U lock:w:0xff:m Reading 1 byte for lock from input file 0xff in 1 section [0, 0] Writing 1 byte (0xFF) to lock, 1 byte written, 1 verified Processing -U efuse:w:{bootloader.extended_fuses}:m Warning: skipping -U efuse:... as memory not defined for part ATmega8 Processing -U hfuse:w:0b11010111:m Reading 1 byte for hfuse from input file 0b11010111 in 1 section [0, 0] Writing 1 byte (0xD7) to hfuse"C:\Users\xxxxx\AppData\Local\Arduino15\packages\MiniCore\tools\avrdude\8.0-arduino.1/bin/avrdude" "-CC:\Users\xxxxx\AppData\Local\Arduino15\packages\MiniCore\tools\avrdude\8.0-arduino.1/etc/avrdude.conf" -v -patmega8 -cstk500v1 -PCOM5 -b19200 "-Uflash:w:C:\Users\xxxxx\AppData\Local\Arduino15\packages\MiniCore\hardware\avr\3.1.1/bootloaders/urboot/atmega8/watchdog_1_s/autobaud/uart0_rxd0_txd1/led+b5/urboot_atmega8_pr_ee_ce.hex:i" -Ulock:w:0xff:m , 1 byte written, 1 verified Processing -U lfuse:w:0b00100100:m Reading 1 byte for lfuse from input file 0b00100100 in 1 section [0, 0] Writing 1 byte (0x24) to lfuse, 1 byte written, 1 verified Avrdude done. Thank you. Avrdude version 8.0-arduino.1 Copyright see https://github.com/avrdudes/avrdude/blob/main/AUTHORS System wide configuration file is C:\Users\xxxxx\AppData\Local\Arduino15\packages\MiniCore\tools\avrdude\8.0-arduino.1\etc\avrdude.conf Using port : COM5 Using programmer : stk500v1 Setting baud rate : 19200 AVR part : ATmega8 Programming modes : SPM, ISP, HVPP Programmer type : STK500 Description : Atmel STK500 v1 HW Version : 2 FW Version : 1.18 Topcard : Unknown Vtarget : 0.0 V Varef : 0.0 V Oscillator : Off SCK period : 0.0 us XTAL frequency : 7.372800 MHz AVR device initialized and ready to accept instructions Device signature = 1E 93 07 (ATmega8, ATmega8A) Auto-erasing chip as flash memory needs programming (-U flash:w:...) specify the -D option to disable this feature Erased chip Processing -U flash:w:C:\Users\xxxxx\AppData\Local\Arduino15\packages\MiniCore\hardware\avr\3.1.1/bootloaders/urboot/atmega8/watchdog_1_s/autobaud/uart0_rxd0_txd1/led+b5/urboot_atmega8_pr_ee_ce.hex:i Reading 372 bytes for flash from input file urboot_atmega8_pr_ee_ce.hex in 2 sections of [0x1e80, 0x1fff]: 6 pages and 12 pad bytes Writing 372 bytes to flash Writing | ################################################## | 100% 0.58s Reading | ################################################## | 100% 0.27s 372 bytes of flash verified Processing -U lock:w:0xff:m Reading 1 byte for lock from input file 0xff in 1 section [0, 0] Writing 1 byte (0xFF) to lock, 1 byte written, 1 verified Avrdude done. Thank you. ``` The flashed bootloader size is 372 bytes, and avrdude verified it successfully. But the high fuse is `D7` which decodes to - BOOTSZ1 **un** programmed (1) - BOOTSZ0 **un** programmed (1) - BOOTRST **un** programmed (1) According to Atmega8A datasheet section 24.8.13 and table 24-6, `BOOTSZ` works like this : - 11 = 128 words = 4 pages - 10 = 256 words = 8 pages - 01 = 512 words = 16 pages - 00 = 1024 words = 32 pages So, if `BOOTSZ[1..0] = 0b11` then the bootloader flash section - is 128 words (256 bytes) - and starts at `0xF80` - goes until the end of the flash memory That is where i am not understanding something 🤨 **Question 1** : how can `avrdude` actually burn a 372 bytes in an actual 256 bytes boot size ? **Question 2** : shouldn't the fuses be `BOOTSZ = 0b10` to get at least 256 words = 512 bytes ? **Question 3** : if not, wouldn't the bootloader be truncated ? ... then how can it still "work" ? Thanks in advance for your insight on this, because i'm not getting it. ---
**Author:** ![hmeijdam](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/hmeijdam/32/218564_2.png) [@hmeijdam](https://forum.arduino.cc/u/hmeijdam)\ **Post date:** [September 23, 2025, 5:31pm UTC](https://forum.arduino.cc/t/atmega8a-bootsz-fuses-smaller-than-urboot-size-using-minicore/1407276/2 "2025-09-23T17:31:16Z")
I don’t know why it writes 372 bytes, but the [bootloader](https://github.com/stefanrueger/urboot) itself is only 250 bytes ![image](https://europe1.discourse-cdn.com/arduino/original/4X/4/7/a/47a67222f3fb5e2e24b244aa112fe18daaff0b29.png) ---
**Author:** ![b707](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/b707/32/854321_2.png) [@b707](https://forum.arduino.cc/u/b707)\ **Post date:** [September 23, 2025, 8:49pm UTC](https://forum.arduino.cc/t/atmega8a-bootsz-fuses-smaller-than-urboot-size-using-minicore/1407276/3 "2025-09-23T20:49:24Z")
> [@nipil](#): > > `Reading 372 bytes for flash from input file urboot_atmega8_pr_ee_ce.hex` The question how those bytes are counted. Perhaps it is a size of the hex file - that always bigger than binary code. ---
**Author:** ![westfw](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/westfw/32/213443_2.png) [@westfw](https://forum.arduino.cc/u/westfw)\ **Post date:** [September 24, 2025, 5:41am UTC](https://forum.arduino.cc/t/atmega8a-bootsz-fuses-smaller-than-urboot-size-using-minicore/1407276/4 "2025-09-24T05:41:23Z")
> [@hmeijdam](#): > > I don’t know why it writes 372 bytes, but the [bootloader](https://github.com/stefanrueger/urboot) itself is only 250 bytes urboot provides several bootloader .hex files. It looks like the "default" one has eeprom and chip-erase support, and is 300+ bytes, even though there are versions slightly less than 256 bytes... ---
**Author:** ![hmeijdam](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/hmeijdam/32/218564_2.png) [@hmeijdam](https://forum.arduino.cc/u/hmeijdam)\ **Post date:** [September 24, 2025, 8:44am UTC](https://forum.arduino.cc/t/atmega8a-bootsz-fuses-smaller-than-urboot-size-using-minicore/1407276/5 "2025-09-24T08:44:08Z")
You are right. Minicore selects the bigger bootloader, which is 372 bytes according Avrdude. ![image](https://europe1.discourse-cdn.com/arduino/original/4X/5/4/c/54c43af4df013160397bf31dd46fd0db234810fd.png) Maybe OP [ask Stefan Rueger](https://github.com/stefanrueger/urboot/discussions), who wrote urboot. ---
**Author:** ![nipil](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/nipil/32/1128570_2.png) [@nipil](https://forum.arduino.cc/u/nipil)\ **Post date:** [September 24, 2025, 9:37am UTC](https://forum.arduino.cc/t/atmega8a-bootsz-fuses-smaller-than-urboot-size-using-minicore/1407276/6 "2025-09-24T09:37:13Z")
@hmeijdam thanks for pointing me to avrdudess, another tool under the belt ! @b707 i checked the HEX file itself, it definitely holds 372 bytes of real information (outside of adresses, record type and checksomes) … so whatever that information is, it’s there. @hmeijdam I found the related readme along the minicore urboot hex files (i didn't see it at first) at [MiniCore/avr/bootloaders/urboot/atmega8/watchdog\_1\_s/autobaud/uart0\_rxd0\_txd1/led+b5 at master · MCUdude/MiniCore · GitHub](https://github.com/MCUdude/MiniCore/tree/master/avr/bootloaders/urboot/atmega8/watchdog_1_s/autobaud/uart0_rxd0_txd1/led%2Bb5) . And according to this statement " **Size:** Bootloader code size _including small table at top end_" ... so the extra bytes ought to be something like the `Interrupt Vector table` at table 12-1 and later in the datasheet. So burning a bootloader might burn a table of the vectors too ! If so, that would make sense, size-wise : - the bootloader (and only itself) is burnt in the BOOTSZ-size boot area - and if sized as shown by avrdudess capture, it would fits in 256 byte (BOOTSZ=0b11) - the vector table (in the hex file) would be at 0x000 (outside BOOTSZ) (see Table 12-2) - its content and size is still to be determined … but that is of lesser importance 🙂 So… i think that rounds it up. Even if it worked perfectly, i could not guess what explained the difference, and now i have a hint. I’ll still browse/ask urboot author page, good advice ! Thanks again, and have a nice day ---
**Author:** ![stefanrueger](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/stefanrueger/32/1013044_2.png) [@stefanrueger](https://forum.arduino.cc/u/stefanrueger)\ **Post date:** [September 26, 2025, 9:25am UTC](https://forum.arduino.cc/t/atmega8a-bootsz-fuses-smaller-than-urboot-size-using-minicore/1407276/7 "2025-09-26T09:25:28Z")
TLDR; the discrepancy between `BOOTSZ=3` and the bootloader size does not matter. The example given is an urboot _vector_ bootloader. Its size can be any multiple of the page size, which is 64 for the ATmega8A, so can be, eg, 256, 320, 384, ... In contrast, ATmega8A's hardware-supported bootloaders only come in sizes of 256, 512, 1024 and 2048 bytes (`BOOTSZ=3...0`). These bootloaders need the fuse setting `BOOTRST=0`, where reset makes the MCU jump to the correct address of the bootloader at the top of flash, ie, to address 8192-256, 8192-512, 8192-1024 or 8192-2048 depending on `BOOTSZ`. Vector bootloaders require `BOOTRST=1` where reset makes the MCU jump to the reset vector at address 0. If the bootloader `.hex` file does not already do so, `avrdude -c urclock` writes to address 0 the correct `rjmp` to the bootloader start. Hence, `BOOTSZ` is irrelevant for the MCU to jump to the bootloader on reset. The magic of bootloaders is that they are allowed to execute the `spm` opcode, which amongst other things can write a flash page. For MCUs that support hardware bootloading (such as the ATmega8A) the `spm` opcode is disabled _outside_ the bootloader section. Urboot bootloaders only ever use _one_ `spm` opcode, which is located near the end of the bootloader code. The smallest bootloader section (`BOOTSZ=3`) for the ATmega8A are the top 256 bytes of flash, which covers the location of the single `spm` urboot opcode. It doesn’t matter that some of the urboot code is below the top 256 bytes bootloader section as it’s regular code (no `spm` there). ---
**Author:** ![nipil](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/nipil/32/1128570_2.png) [@nipil](https://forum.arduino.cc/u/nipil)\ **Post date:** [September 26, 2025, 4:33pm UTC](https://forum.arduino.cc/t/atmega8a-bootsz-fuses-smaller-than-urboot-size-using-minicore/1407276/8 "2025-09-26T16:33:29Z")
I was planning to post a summary here but Stefan beat me to it And with a synthetic, spot-on summary of the awesome [discussion](https://github.com/stefanrueger/urboot/discussions/44) on his repo. Great stuff, very detailed and instructive 👍 Thanks a lot everyone. ---
**Author:** ![system](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/system/32/1140315_2.png) [@system](https://forum.arduino.cc/u/system)\ **Post date:** [March 25, 2026, 4:33pm UTC](https://forum.arduino.cc/t/atmega8a-bootsz-fuses-smaller-than-urboot-size-using-minicore/1407276/9 "2026-03-25T16:33:40Z")
Closed on March 25, 2026, 4:33pm UTC This topic was automatically closed 180 days after the last reply. New replies are no longer allowed.